X-Git-Url: https://git.rrq.au/?a=blobdiff_plain;f=fusefile.c;h=01f4c58400f0d4a2197797ddebb6a415e27b808a;hb=710729d1704db38b5facd708acc3267086701711;hp=55ba86aae686986003fefb3b6e083abfb44d4b75;hpb=78348bed8433e7b7429ff4a0233b43ba614ac28d;p=rrq%2Ffusefile.git

diff --git a/fusefile.c b/fusefile.c
index 55ba86a..01f4c58 100644
--- a/fusefile.c
+++ b/fusefile.c
@@ -34,18 +34,23 @@
 #include <time.h>
 #include <errno.h>
 
+struct Region {
+    off_t beg;
+    off_t end;
+};
+
 struct Source {
     char *filename;
     ssize_t from;
     ssize_t to;
     ssize_t start; // starting position in concatenated file
     int fd;
+    int dirty;
 };
 
 static struct {
     struct Source *array;
     int count;
-    int limit;
     ssize_t size;
 } sources;
 
@@ -54,67 +59,235 @@ static struct {
     time_t mtime;
     time_t ctime;
 } times;
-    
-#define SOURCEARRAYP(i) ((void*)&sources.array[ i ])
 
 /**
- * Holds info about scratch pad file for 'write' events.
+ * Overlay
  */
 static struct {
-    char *filename;
-    int fd;
-} pad;
+    struct Source source;
+    struct Region *table;
+    size_t count;
+    size_t limit;
+} overlay;
 
-#if DEBUG
-static void print_source(struct Source *p) {
-    fprintf( stderr, "%p { %s, %ld, %ld, %ld, %d }\n",
-	     p, p->filename, p->from, p->to, p->start, p->fd );
+static void usage();
+
+/**
+ * Find the nearest overlay.table region below pos. Returns the index,
+ * or -1 if there is none, i.e. pos < overlay.table[0].
+ */
+static ssize_t overlay_prior_fragment(off_t pos) {
+    size_t lo = 0, hi = overlay.count;
+    while ( lo < hi ) {
+	size_t m = ( lo + hi ) / 2;
+	if ( m == lo ) {
+	    return overlay.table[m].beg <= pos? m : -1;
+	}
+	if ( overlay.table[m].beg <= pos ) {
+	    lo = m;
+	} else {
+	    hi = m;
+	}
+    }
+    return -1;
 }
-#endif
 
-// Scan the source specification, and return the length of the
-// inclusion. "filename/from,to"
-// filename
-// filename/from
-// filename/-to
-// filename/from-to
-static size_t scan_source(char *in,struct Source *p) {
-    int e = strlen( in );
-    int i = e-1;
-    int s = -1;
-    int m = -1;
-    // scan for last '/' and last '-'
-    for ( ; i >= 0; i-- ) {
-	if ( in[i] == '/' ) {
-	    s = i;
-	    break;
+/**
+ * Save the entry count for overlay.table as 64-bit integer
+ * immediately following the overlay content at the index
+ * corresponding to the fused file size.
+ */
+static void overlay_save_count() {
+    lseek( overlay.source.fd, overlay.source.to, SEEK_SET );
+    size_t size = sizeof( overlay.count );
+    char *p = (char *) &overlay.count ;
+    while ( size > 0 ) {
+	size_t n = write( overlay.source.fd, p, size );
+	if ( n < 0 ) {
+	    perror( overlay.source.filename );
+	    exit( 1 );
 	}
-	if ( in[i] == '-' ) {
-	    m = i;
+	size -= n;
+	p += n;
+    }
+    if ( overlay.source.dirty++ > 1000 ) {
+	fsync( overlay.source.fd );
+	overlay.source.dirty = 0;
+    }
+}
+
+/**
+ * Update the on-disk cache of overlay.table between the given
+ * indexes. The table is laid out immediately following the table
+ * count with each region saved as two 64-bit unsigned integers.
+ */
+static void overlay_save_table(size_t lo,size_t hi) {
+    char *p = (char *) &overlay.table[ lo ];
+    size_t pos =  overlay.source.to + sizeof( overlay.count ) +
+	lo * sizeof( struct Region );
+    size_t size = ( hi - lo ) * sizeof( struct Region );
+    if ( pos != lseek( overlay.source.fd, pos, SEEK_SET ) ) {
+	fprintf( stderr, "%s: seek error\n", overlay.source.filename );
+	exit( 1 );
+    }
+    while ( size > 0 ) {
+	size_t n = write( overlay.source.fd, p, size );
+	if ( n < 0 ) {
+	    perror( overlay.source.filename );
+	    exit( 1 );
 	}
+	size -= n;
+	p += n;
+    }
+    if ( overlay.source.dirty++ > 1000 ) {
+	fsync( overlay.source.fd );
+	overlay.source.dirty = 0;
+    }
+}
+
+/**
+ * Insert a new region at index p, with previous portion [p,count]
+ * moved up to make space.
+ */
+static void overlay_insert(size_t p,off_t beg,off_t end) {
+    size_t bytes;
+    // Grow the table if needed
+    if ( overlay.count >= overlay.limit ) {
+	overlay.limit = overlay.count + 10;
+	bytes = overlay.limit * sizeof( struct Region );
+	overlay.table = overlay.table?
+	    realloc( overlay.table, bytes ) : malloc( bytes );
+    }
+    bytes = ( overlay.count++ - p ) * sizeof( struct Region );
+    if ( bytes ) {
+	memmove( (char*) &overlay.table[ p+1 ],
+		 (char*) &overlay.table[ p ],
+		 bytes );
+    }
+    overlay.table[ p ].beg = beg;
+    overlay.table[ p ].end = end;
+    overlay_save_count();
+}
+
+/**
+ * Delete the region entry at p by moving the portion [p+1,count]
+ * down.
+ */
+static void overlay_delete(size_t p) {
+    size_t bytes = ( --overlay.count - p ) * sizeof( struct Region );
+    if ( bytes ) {
+	memmove( (char*) &overlay.table[ p ],
+		 (char*) &overlay.table[ p+1 ],
+		 bytes );
     }
+}
+
+/**
+ * Mark the given region as updated, i.e. written to the overlay. The
+ * mark region may attach to prior marked regions ro be a new,
+ * separate region. If attaching, it causes the prior regions to
+ * expand and the table adjusted by deleting any regions that become
+ * fully contained in other regions.
+ */
+static void overlay_mark(off_t beg,off_t end) {
+#if DEBUG
+    fprintf( stderr, "overlay_mark( %ld, %ld )\n", beg, end );
+#endif
+    int deleted = 0;
+    ssize_t q;
+    ssize_t p = overlay_prior_fragment( beg );
+    // p is the nearest region below or at beg (or -1)
+    if ( p >= 0 && beg <= overlay.table[p].end ) {
+	// p overlaps mark region
+	if ( end <= overlay.table[p].end ) {
+	    // region p covers mark region already
+#if DEBUG
+	    fprintf( stderr, "overlay covering ( %ld %ld )\n",
+		     overlay.table[p].beg, overlay.table[p].end );
+#endif
+	    return;
+	}
+	// the new mark region extends region p
+	overlay.table[p].end = end;
+	q = p+1;
+	while ( q < overlay.count &&
+		overlay.table[q].beg <= overlay.table[p].end ) {
+	    // Extended region merges with subsequent region
+	    if ( overlay.table[p].end < overlay.table[q].end ) {
+		overlay.table[p].end = overlay.table[q].end;
+	    }
+	    overlay_delete( q );
+	    deleted++;
+	}
+	if ( deleted ) {
+	    overlay_save_count();
+	    q = overlay.count;
+	}
+	overlay_save_table( p, q );
 #if DEBUG
-    fprintf( stderr, "m=%d s=%d\n", m, s );
+	fprintf( stderr, "overlay expand ( %ld %ld ) deleted %d\n",
+		 overlay.table[p].beg, overlay.table[p].end, deleted );
 #endif
-    // Copy the filename, and set from and to
-    p->filename = strndup( in, ( s < 0 )? e : s );
-    struct stat buf;
-    if ( stat( p->filename, & buf ) ) {
-	perror( p->filename );
-	return 1; 
+	return;
     }
-    p->from = ( s < 0 )? 0 : atol( in+s+1 );
-    if ( p->from < 0 ) {
-	p->from = 0;
+    // The prior region p does not expand into new mark region
+    p++; // subsequent region 
+    if ( p >= overlay.count || end < overlay.table[p].beg ) {
+	// New mark region is a separate region at p
+	overlay_insert( p, beg, end );
+#if DEBUG
+	fprintf( stderr, "overlay new ( %ld %ld )\n",
+		 overlay.table[p].beg, overlay.table[p].end );
+#endif
+	overlay_save_table( p, overlay.count );
+	return;
+    }
+    // New marks start before and overlap with region p => change p
+    // and handle any subsequent regions being covered
+    overlay.table[p].beg = beg;
+    q = p+1;
+    if ( overlay.table[p].end < end ) {
+	overlay.table[p].end = end;
+	while ( q < overlay.count &&
+		overlay.table[q].beg <= overlay.table[p].end ) {
+	    if ( overlay.table[p].end < overlay.table[q].end ) {
+		overlay.table[p].end = overlay.table[q].end;
+	    }
+	    overlay_delete( q );
+	    deleted++;
+	}
+	if ( deleted ) {
+	    overlay_save_count();
+	    q = overlay.count;
+	}
     }
+    overlay_save_table( p, q );
 #if DEBUG
-    fprintf( stderr, "p->from=%ld\n", p->from );
+    fprintf( stderr, "overlay before ( %ld %ld ) deleted %d\n",
+	     overlay.table[p].beg, overlay.table[p].end, deleted );
 #endif
-    p->to = ( m < 0 )? buf.st_size : atol( in+m+1 );
-    if ( p->from > p->to || p->to > buf.st_size ) {
-	return 1;
+}
+
+static void setup_overlay(char *filename) {
+    overlay.source.filename = filename;
+    overlay.source.fd = open( filename, O_RDWR | O_CREAT, S_IRUSR | S_IWUSR );
+    if ( overlay.source.fd < 0 ) {
+	perror( filename );
+	usage();
     }
-    return 0;
+}
+
+#if DEBUG
+static void print_source(struct Source *p) {
+    fprintf( stderr, "%p { %s, %ld, %ld, %ld, %d }\n",
+	     p, p->filename, p->from, p->to, p->start, p-> fd );
+}
+#endif
+
+static char *range;
+static unsigned int c;
+static int RANGE(int s,int n ) {
+    return ( s == n ) && *(range+c) == 0;
 }
 
 static int setup_sources(char **argv,int i,int n) {
@@ -123,22 +296,78 @@ static int setup_sources(char **argv,int i,int n) {
 	return 1;
     }
     sources.count = n;
-    sources.limit = n;
     int j = 0;
     sources.size = 0;
     for ( ; j < n; i++, j++ ) {
+	struct stat filestat;
 	struct Source *p = sources.array + j;
-	if ( scan_source( argv[i], p ) ) {
-	    // should free everything malloc-ed
-	    return 1;
+	// Open the fragment file rw if possible, else ro
+	range = strrchr( argv[i], '/' ); // last '/'
+	p->filename = range? strndup( argv[i], range - argv[i] ) : argv[i];
+	p->fd = open( p->filename, O_RDWR );
+	int rdonly = 0;
+	if ( p->fd < 0 ) {
+	    rdonly = 1;
+	    p->fd = open( p->filename, O_RDONLY );
 	}
-	p->start = sources.size;
-	sources.size += p->to - p->from;
-	p->fd = open( p->filename, O_RDONLY );
 	if ( p->fd < 0 ) {
 	    perror( p->filename );
+	    return 1; // Error return
+	}
+	if ( stat( p->filename, &filestat ) ) {
+	    perror( p->filename );
+	    return 1; 
+	}
+	if ( rdonly ) {
+	    fprintf( stderr, "** %s opened read-only\n", p->filename );
+	}
+	p->from = 0;
+	p->to = filestat.st_size;
+	// Process any range variation
+	if ( range && *(++range) ) {
+	    int a,b;
+	    if ( 0 ) {
+	    } else if ( RANGE( sscanf( range, "%d:%d%n", &a, &b, &c ), 2 )) {
+		p->from = ( a < 0 )? ( p->to + a ) : a;
+		p->to = ( b < 0 )? ( p->to + b ) : b;
+	    } else if ( RANGE( sscanf( range, "%d+%d%n", &a, &b, &c ), 2 )) {
+		p->from = ( a < 0 )? ( p->to + a ) : a;
+		p->to = ( ( b < 0 )? p->to : p->from ) + b;
+	    } else if ( RANGE( sscanf( range, "%d+%n", &a, &c ), 1 )) {
+		p->from = ( a < 0 )? ( p->to + a ) : a;
+	    } else if ( RANGE( sscanf( range, ":%d%n", &b, &c ), 1 )) {
+		p->to = ( b < 0 )? ( p->to + b ) : b;
+	    } else if ( RANGE( sscanf( range, "%d:%n", &a, &c ), 1 )) {
+		p->from = ( a < 0 )? ( p->to + a ) : a;
+	    } else if ( RANGE( sscanf( range, "%d%n", &a, &c ), 1 )) {
+		if ( a >= 0 ) {
+		    p->from = a;
+		} else {
+		    p->from = p->to + a;
+		}
+	    } else if ( RANGE( sscanf( range, ":%n", &c), 0 ) ) {
+		// to end from start
+	    } else {
+		fprintf( stderr, "** BAD RANGE: %s\n", argv[i] );
+		return 1;
+	    }
+	}
+	if ( ( filestat.st_mode &  S_IFMT ) == S_IFCHR ) {
+	    filestat.st_size = p->to; // Pretend size of character device
+	}
+	if ( p->from < 0 ) {
+	    p->from = 0;
+	}
+	if ( p->to > filestat.st_size ) {
+	    p->to = filestat.st_size;
+	}
+	if ( p->from >= p->to || p->from >= filestat.st_size ) {
+	    fprintf( stderr, "** BAD RANGE: %s [%ld:%ld]\n",
+		     argv[i], p->from, p->to );
 	    return 1;
 	}
+	p->start = sources.size; // the fusefile position of fragment
+	sources.size += p->to - p->from;
 #if DEBUG
 	print_source( p );
 #endif
@@ -157,10 +386,7 @@ static int fusefile_getattr(const char *path,struct stat *stbuf) {
     fprintf( stderr, "getattr %ld\n", sources.size );
 #endif
     memset( stbuf, 0, sizeof( struct stat ) );
-    stbuf->st_mode = S_IFREG | 0444; // Hmmm
-    if ( pad.filename ) {
-	stbuf->st_mode |= 0200;
-    }
+    stbuf->st_mode = S_IFREG | 0644; // Hmmm
     stbuf->st_nlink = 1;
     stbuf->st_size = sources.size;
     stbuf->st_atime = times.atime;
@@ -171,6 +397,13 @@ static int fusefile_getattr(const char *path,struct stat *stbuf) {
     return 0;
 }
 
+static int fusefile_chmod(const char *path,mode_t m) {
+#if DEBUG
+    fprintf( stderr, "fusefile_chmod( %s, %d )\n", path, m );
+#endif
+    return -1;
+}
+
 static int fusefile_open(const char *path,struct fuse_file_info *fi) {
 #if DEBUG
     fprintf( stderr, "fusefile_open( %s, %d )\n", path, fi->flags );
@@ -187,108 +420,86 @@ static int fusefile_open(const char *path,struct fuse_file_info *fi) {
 static int find_source(off_t offset) {
     int lo = 0;
     int hi = sources.count;
-    if ( offset > sources.size ) {
+    if ( offset >= sources.size ) {
 	return -1;
     }
+#if DEBUG
+    fprintf( stderr, "find_source( %ld )\n", offset );
+#endif
     while ( lo + 1 < hi ) {
 	int m = ( lo + hi ) / 2;
 	if ( offset < sources.array[ m ].start ) {
+#if DEBUG
+	    fprintf( stderr, "  offset < [%d].start: %ld\n",
+		     m, sources.array[ m ].start );
+#endif
 	    hi = m;
 	} else {
+#if DEBUG
+	    fprintf( stderr, "  offset >= [%d].start: %ld\n",
+		     m, sources.array[ m ].start );
+#endif
 	    lo = m;
 	}
     }
+#if DEBUG
+    fprintf( stderr, "found %d\n", lo );
+#endif
     return lo;
 }
 
-/**
- * Insert a source fragment description into the table at <off>.
- */
-static int insert_source(struct Source *source,size_t off) {
-    int index = find_source( off );
-    int i;
-    // Ensure at least 5 "free" Sources in <source.array>
-    // and allocate space for 20 new otherwise.
-    if ( sources.count + 5 > sources.limit ) {
-	size_t size = sources.limit + 20;
-	struct Source *new = realloc(
-	    sources.array, size * sizeof( struct Source ) );
-	if ( new == 0 ) {
-	    return -1;
-	}
-	sources.array = new;
-	sources.limit = size;
-    }
-#if DEBUG
-    fprintf( stderr, "index=%d\n", index );
-#endif
-    if ( index < sources.count ) {
-	ssize_t b = ( sources.count - index ) * sizeof(struct Source);
-#if DEBUG
-    fprintf( stderr, "b=%ld\n", b );
-#endif
-	if ( sources.array[ index ].start < off ) {
-	    // Split the <index> record at <off>
-	    // and adjust index
-	    memcpy( SOURCEARRAYP( index+2 ), SOURCEARRAYP( index ), b );
-	    sources.count += 2;
-	    b = off - sources.array[ index ].start;
-	    sources.array[ index + 2 ].from += b; // adjust tail fragment
-	    sources.array[ index++ ].to = b; // adjust head fragment
+static int overlay_merge(char *buf,off_t beg,off_t end) {
 #if DEBUG
-	    print_source( &sources.array[ index-1 ] );
-	    print_source( &sources.array[ index ] );
-	    print_source( &sources.array[ index+1 ] );
-	    fprintf( stderr, "---\n");
+    fprintf( stderr, "merge %ld %ld\n", beg, end );
 #endif
-	} else {
-	    // Insert the new source at <index>
-	    memcpy( SOURCEARRAYP( index+1 ), SOURCEARRAYP( index ), b );
-	    sources.count += 1;
+    // Find nearest overlay data before or at beg
+    ssize_t p = overlay_prior_fragment( beg );
+    if ( p < 0 ) {
+	p = 0;
+    }
+    for ( ; p < overlay.count && overlay.table[p].beg < end; p++ ) {
+	if ( overlay.table[p].end < beg ) {
+	    continue;
+	}
+	if ( overlay.table[p].beg > beg ) {
+	    size_t delta = overlay.table[p].beg - beg;
+	    buf += delta;
+	    beg += delta;
+	}
+	size_t size = ( overlay.table[p].end <= end )?
+	    ( overlay.table[p].end - beg ) : ( end - beg ); 
+	lseek( overlay.source.fd, beg, SEEK_SET );
+	while ( size > 0 ) {
+	    size_t n = read( overlay.source.fd, buf, size );
+	    size -= n;
+	    buf += n;
+	    beg += n; //
 	}
-    } else {
-	// Append the new source to <sources> (at <index>)
-	sources.count += 1;
-    }
-    sources.array[ index ].filename = source->filename;
-    sources.array[ index ].fd = source->fd;
-    sources.array[ index ].from = source->from;
-    sources.array[ index ].to = source->to;
-    for ( i = index; i < sources.count; i++ ) {
-	sources.array[ i ].start = off;
-	off += sources.array[ i ].to - sources.array[ i ].from;
-#if DEBUG
-	print_source( &sources.array[ i ] );
-#endif
     }
-    sources.size = off;
-#if DEBUG
-    fprintf( stderr, "count=%d size=%ld\n", sources.count, sources.size );
-#endif
-    return index;
+    return 0;
 }
 
-// Read <size> bytes from <off> in file
+// Read <size> bytes from <offset> in file
 static int fusefile_read(const char *path, char *buf, size_t size,
 			 off_t off, struct fuse_file_info *fi)
 {
 #if DEBUG
     fprintf( stderr, "fusefile_read( %s )\n", path );
 #endif
-    if ( strcmp( path, "/" ) != 0 ) {
+    if( strcmp( path, "/" ) != 0 ) {
 	return -ENOENT;
     }
 #if DEBUG
     fprintf( stderr, "read %ld %ld\n", off, size );
 #endif
-    size_t rr = 0;
+    size_t rr = 0; // total reading
     while ( size > 0 ) {
 #if DEBUG
-	fprintf( stderr, "find_source %ld %ld\n", off, size );
+	fprintf( stderr, "  find_source %ld %ld\n", off, size );
 #endif
 	int i = find_source( off );
 	if ( i < 0 ) {
-	    return -ENOENT;
+	    return ( off == sources.size )? rr : -ENOENT;
 	}
 	if ( sources.array[i].fd < 0 ) {
 	    return -ENOENT;
@@ -296,21 +507,30 @@ static int fusefile_read(const char *path, char *buf, size_t size,
 #if DEBUG
 	print_source( &sources.array[i] );
 #endif
+	times.atime = time( 0 );
 	size_t b = off - sources.array[i].start + sources.array[i].from;
 	size_t n = sources.array[i].to - b;
 	if ( n > size ) {
 	    n = size;
 	}
+	if ( sources.array[i].dirty ) {
+	    fsync( sources.array[i].fd );
+	    sources.array[i].dirty = 0;
+	}
+#if DEBUG
+	fprintf( stderr, "  seek fd=%d to %ld\n", sources.array[i].fd, b );
+#endif
 	if ( lseek( sources.array[i].fd, b, SEEK_SET ) < 0 ) {
 	    perror( sources.array[i].filename );
 	    return -ENOENT;
 	}
 #if DEBUG
-	fprintf( stderr, "get %ld bytes at %ld\n", n, rr );
+	fprintf( stderr, "  now read %ld from fd=%d\n",
+		 n, sources.array[i].fd );
 #endif
 	ssize_t r = read( sources.array[i].fd, buf + rr, n );
 #if DEBUG
-	fprintf( stderr, "got %ld bytes\n", r );
+	fprintf( stderr, "  got %ld bytes\n", r );
 #endif
 	if ( r < 0 ) {
 	    perror( sources.array[i].filename );
@@ -319,28 +539,109 @@ static int fusefile_read(const char *path, char *buf, size_t size,
 	if ( r == 0 ) {
 	    break;
 	}
+	if ( overlay.source.filename ) {
+	    if ( overlay.source.dirty ) {
+		fsync( overlay.source.fd );
+		overlay.source.dirty = 0;
+	    }
+	    int x = overlay_merge( buf + rr, off + rr, off + rr + r );
+	    if ( x ) {
+		return x;
+	    }
+	}
 	rr += r;
 	off += r;
 	size -= r;
     }
-    times.atime = time( 0 );
+#if DEBUG
+    fprintf( stderr, "  total reading %ld bytes\n", rr );
+#endif
     return rr;
 }
 
 /**
- * Write a full block of data.
+ * Poll for IO readiness.
  */
-static int write_block(int fd,const char *buf,size_t size) {
-    size_t orig = size;
+int fusefile_poll(const char *path, struct fuse_file_info *fi,
+		   struct fuse_pollhandle *ph, unsigned *reventsp )
+{
+#if DEBUG
+    fprintf( stderr, "fusefile_poll( %s ) %p %d\n", path, ph, *reventsp );
+#endif
+    if( strcmp( path, "/" ) != 0 ) {
+	return -ENOENT;
+    }
+    if ( ph ) {
+	return fuse_notify_poll( ph );
+    }
+    return 0;
+}
+
+static void overlay_load() {
+    lseek( overlay.source.fd, overlay.source.to, SEEK_SET );
+    size_t x = 0;
+    size_t size = sizeof( overlay.count );
+    if ( read( overlay.source.fd, &x, size ) != size ) {
+	return;
+    }
+#if DEBUG
+    fprintf( stderr, "overlay: %s with %ld regions\n",
+	     overlay.source.filename, x );
+#endif
+    struct Region f = { 0, 0 };
+    size = sizeof( struct Region );
+    while ( x-- > 0 ) {
+	if ( read( overlay.source.fd, &f, size ) != size ) {
+	    fprintf( stderr, "%s: bad meta data\n", overlay.source.filename );
+	    exit( 1 );
+	}
+#if DEBUG
+	fprintf( stderr, "overlay region: %ld %ld\n", f.beg, f.end );
+#endif
+	overlay_mark( f.beg, f.end );
+    }
+}
+
+/**
+ * Write a full block of data over the sources at the offset
+ */
+static int write_block(off_t off,const char *buf,size_t size) {
+#if DEBUG
+    fprintf( stderr, "write_block( %ld, ?, %ld )\n", off, size );
+#endif
+    if ( overlay.source.filename ) {
+	overlay_mark( off, off + size ); // Mark region as written
+    }
     while ( size > 0 ) {
-	ssize_t n = write( fd, buf, size );
-	if ( n <= 0 ) {
-	    return n;
+	int index = find_source( off ); // index of source file
+	if ( index < 0 ) {
+	    return -EIO; // past EOF
+	}
+	struct Source *source = overlay.source.filename?
+	    &overlay.source :  &sources.array[ index ];
+	off_t from = off - source->start + source->from;
+	off_t max = source->to - from;
+	if ( lseek( source->fd, from, SEEK_SET ) < 0 ) {
+	    return -EIO;
+	}
+	ssize_t todo = ( size < max )? size : max;
+	while ( todo > 0 ) {
+	    times.mtime = time( 0 );
+	    ssize_t n = write( source->fd, buf, todo );
+	    if ( n <= 0 ) {
+		return -EIO; // Something wrong
+	    }
+	    buf += n;
+	    todo -= n;
+	    size -= n;
+	    off += n;
+	}
+	if ( source->dirty++ >= 1000 ) {
+	    fsync( source->fd );
+	    source->dirty = 0;
 	}
-	buf += n;
-	size -= n;
     }
-    return orig;
+    return 0;
 }
 
 static int fusefile_write_buf(const char *path, struct fuse_bufvec *buf,
@@ -351,38 +652,9 @@ static int fusefile_write_buf(const char *path, struct fuse_bufvec *buf,
     if ( strcmp( path, "/" ) != 0 ) {
 	return -ENOENT;
     }
-    
-    // Ensure a pad was nominated
-    if ( pad.filename == 0 ) {
-	return 1;
-    }
 
-    // Determine total size
     size_t size = 0;
     int i;
-#if DEBUG
-    fprintf( stderr, "count = %ld\n", buf->count );
-#endif
-    for ( i = 0; i < buf->count; i++ ) {
-	struct fuse_buf *p = &buf->buf[ i ];
-	size += p->size;
-    }
-    static char meta[ 100 ];
-    sprintf( meta, "%ld\n%ld\n", off, size );
-#if DEBUG
-    fprintf( stderr, "meta( %ld %ld )\n", off, size );
-#endif
-    if ( write_block( pad.fd, meta, strlen( meta ) ) <= 0 ) {
-	perror( pad.filename );
-	return -EIO;
-    }
-    struct Source source = {
-	.filename = pad.filename,
-	.fd = pad.fd,
-	.from = lseek( pad.fd, 0, SEEK_END ),
-	.to = 0,
-	.start = 0
-    };
     for ( i = 0; i < buf->count; i++ ) {
 	struct fuse_buf *p = &buf->buf[i];
 	if ( p->flags & FUSE_BUF_IS_FD ) {
@@ -390,24 +662,20 @@ static int fusefile_write_buf(const char *path, struct fuse_bufvec *buf,
 	    fprintf( stderr, "Content held in a file ... HELP!!\n" );
 #endif
 	    return -EIO;
-	} else {
-	    ssize_t n = write_block( pad.fd, (char*) p->mem, p->size );
-	    if ( n != p->size ) {
-		return -EIO;
-	    }
 	}
+	if ( write_block( off, (char*) p->mem, p->size ) < 0 ) {
+	    return -EIO;
+	}
+	size += p->size;
     }
-    source.to = source.from + size;
-    insert_source( &source, off );
-    times.mtime = time( 0 );
+#if DEBUG
+    fprintf( stderr, "fusefile_write_buf written %ld\n", size );
+#endif
     return size;
 }
 
 /**
- * Insert a fragment at <off>. The data is appended to the pad file,
- * and a descriptor is inserted; the fragment containing <off> is
- * first split, unless <off> is at its start, and then new fragment
- * descriptor is inserted.
+ * Write a fragment at <off>. This overwrites files.
  */
 static int fusefile_write(const char *path, const char *buf, size_t size,
 			  off_t off, struct fuse_file_info *fi)
@@ -419,30 +687,9 @@ static int fusefile_write(const char *path, const char *buf, size_t size,
 	return -ENOENT;
     }
 
-    // Ensure a pad was nominated
-    if ( pad.filename == 0 ) {
-	return 1;
-    }
-    static char meta[ 100 ];
-    sprintf( meta, "%ld\n%ld\n", off, size );
-    if ( write_block( pad.fd, meta, strlen( meta ) ) <= 0 ) {
-	perror( pad.filename );
+    if ( write_block( off, buf, size ) < 0 ) {
 	return -EIO;
     }
-    struct Source source = {
-	.filename = pad.filename,
-	.fd = pad.fd,
-	.from = lseek( pad.fd, 0, SEEK_END ),
-	.to = 0,
-	.start = 0
-    };
-    ssize_t n = write_block( pad.fd, buf, size );
-    if ( n != size ) {
-	return n;
-    }
-    source.to = source.from + size;
-    insert_source( &source, off );
-    times.mtime = time( 0 );
     return size;
 }
 
@@ -456,6 +703,20 @@ static void fusefile_destroy(void *data) {
     }
 }
 
+static void fsync_all_dirty() {
+    int i = 0;
+    for ( ; i < sources.count; i++ ) {
+	if ( sources.array[i].dirty ) {
+	    fsync( sources.array[i].fd );
+	    sources.array[i].dirty = 0;
+	}
+    }
+    if ( overlay.source.filename && overlay.source.dirty ) {
+	fsync( overlay.source.fd );
+	overlay.source.dirty = 0;
+    }
+}
+
 static int fusefile_flush(const char *path, struct fuse_file_info *info) {
 #if DEBUG
     fprintf( stderr, "fusefile_flush( %s )\n", path );
@@ -463,6 +724,7 @@ static int fusefile_flush(const char *path, struct fuse_file_info *info) {
     if ( strcmp( path, "/" ) != 0 ) {
 	return -ENOENT;
     }
+    fsync_all_dirty();
     return 0;
 }
 
@@ -483,6 +745,7 @@ static int fusefile_fsync(const char *path, int x, struct fuse_file_info *fi) {
     if ( strcmp( path, "/" ) != 0 ) {
 	return -ENOENT;
     }
+    fsync_all_dirty();
     return 0;
 }
 
@@ -499,10 +762,25 @@ static int fusefile_truncate(const char *path, off_t len) {
     return -EIO;
 }
 
+void *fusefile_init(struct fuse_conn_info *fci) {
+#if DEBUG
+    fprintf( stderr, "fusefile_init( %d, %d )\n", fci->async_read, fci->want );
+#endif
+    // Disable asynchronous reading
+    fci->async_read = 0;
+    fci->want &= ~FUSE_CAP_ASYNC_READ;
+#if DEBUG
+    fprintf( stderr, "fusefile_init( %d, %d )\n", fci->async_read, fci->want );
+#endif
+    return 0;
+}
+
 static struct fuse_operations fusefile_oper = {
     .getattr = fusefile_getattr,
+    .chmod = fusefile_chmod,
     .open = fusefile_open,
     .read = fusefile_read,
+    .poll = fusefile_poll,
     .write = fusefile_write,
     .write_buf = fusefile_write_buf,
     .destroy = fusefile_destroy,
@@ -512,13 +790,13 @@ static struct fuse_operations fusefile_oper = {
     .truncate = fusefile_truncate,
     //.truncate = fusefile_truncate,
     //.release = fusefile_release,
-    //void *(*init) (struct fuse_conn_info *conn);
+    .init = fusefile_init,
 };
 
 static void usage() {
     char *usage =
 "Usage: fusefile [ <fuse options> ] <mount> <file/from-to> ... \n"
-"Mounts a virtual, read-only file that is a concatenation of file fragments\n"
+"Mounts a virtual, file that is a concatenation of file fragments\n"
 	;
     fprintf( stderr, "%s", usage );
     exit( 1 );
@@ -526,15 +804,31 @@ static void usage() {
 
 /**
  * Set up the arguments for the fuse_main call, adding our own.
+ * argv[argc] is the mount point argument
  */
 static int setup_argv(int argc,char ***argv) {
-    int n = argc + 1;
-    char **out = calloc( n--, sizeof( char* ) );
-    memcpy( (void*) out, (void*) (*argv), argc-- * sizeof( char* ) ); 
-    out[ n++ ] = out[ argc ]; // mount point
-    out[ argc++ ] = "-odefault_permissions";
+    // note: (*argv)[ argc ] is the mount point argument
+    char *OURS[] = {
+	"-odefault_permissions",
+	(*argv)[ argc ]
+    };
+#define OURSN ( sizeof( OURS ) / sizeof( char* ) )
+    int N = argc + OURSN;
+    // Allocate new arg array plus terminating null pointer
+    char **out = malloc( ( N + 1 ) * sizeof( char* ) ); 
+    int i;
+    for ( i = 0; i < argc; i++ ) {
+	out[ i ] = (*argv)[i];
+	//fprintf( stderr, " %s", out[ i ] );
+    }
+    for ( i = 0; i < OURSN; i++ ) {
+	out[ argc + i ] = OURS[i];
+	//fprintf( stderr, " %s", out[ i ] );
+    }
+    out[ N ] = 0;
+    //fprintf( stderr, "\n" );
     (*argv) = out;
-    return n;
+    return N; // Don't include the terminating null pointer
 }
 
 /**
@@ -559,24 +853,24 @@ int main(int argc, char *argv[])
     if ( i > argc - 2 ) { // At least mount point plus one source
 	usage();
     }
-    mnt = argv[ i++ ]; // First non-option argument is the mount pount
     fuseargc = i;
-    if ( strncmp( argv[ i ], "pad=", 4 ) == 0 ) {
-	// First argument is the pad, if any, signaled with "pad=" prefix
-	pad.filename = argv[ i++ ] + 4; // (also move arg index)
-#if DEBUG
-	fprintf( stderr, "scratch pad=%s\n", pad.filename );
-#endif
-	pad.fd = open( pad.filename, O_RDWR | O_CREAT, 0600 );
-	if ( pad.fd < 0 ) {
-	    perror( pad.filename );
-	    exit( errno );
+    mnt = argv[ i++ ]; // First non-option argument is the mount pount
+    char *overlaytag = "-overlay:";
+    int overlaytagsize = strlen( overlaytag );
+    if ( strncmp( argv[i], overlaytag, overlaytagsize ) == 0 ) {
+	// consume "-overlay:filename"
+	setup_overlay( argv[i++] + overlaytagsize ); // Need a writable file
+	if ( i >= argc ) {
+	    usage();
 	}
-	lseek( pad.fd, 0, SEEK_END ); // initial seek
     }
     if ( setup_sources( argv, i, argc-i ) ) {
 	return 1;
     }
+    if ( overlay.source.filename ) {
+	overlay.source.to = sources.size; // Register total size.
+	overlay_load();
+    }
     if ( stat( mnt, &stbuf ) == -1 ) {
 	int fd = open( mnt, O_CREAT | O_RDWR, S_IRUSR | S_IWUSR );
 	if ( fd < 0 ) {
@@ -598,6 +892,16 @@ int main(int argc, char *argv[])
 	times.ctime = stbuf.st_ctime;
     }
 
+    {
+	int fd = open( mnt, O_RDWR, S_IRUSR | S_IWUSR );
+	if ( fd < 0 ) {
+	    perror( mnt );
+	    return 1;
+	}
+	if ( lseek( fd, sources.size, SEEK_SET ) < 0 ) {
+	    return -EIO;
+	}
+    }
     fuseargc = setup_argv( fuseargc, &argv );
     struct fuse_args args = FUSE_ARGS_INIT( fuseargc, argv );
     if ( fuse_parse_cmdline( &args, &mnt, &mt, &fg ) ) {