X-Git-Url: https://git.rrq.au/?a=blobdiff_plain;f=fusefile.c;h=30ec7fa284c7ce1f330daaede1de2fb1835ee9b5;hb=6c93ae8d4f1955020adedfa67a43478dba854ab0;hp=476ca9d8fb1ffa847e094150b44223a816ed7603;hpb=a83a2cd9258ecb0420a019bb5d4cd32db8a444ac;p=rrq%2Ffusefile.git

diff --git a/fusefile.c b/fusefile.c
index 476ca9d..30ec7fa 100644
--- a/fusefile.c
+++ b/fusefile.c
@@ -34,12 +34,18 @@
 #include <time.h>
 #include <errno.h>
 
+struct Region {
+    off_t pos;
+    size_t size;
+};
+
 struct Source {
     char *filename;
     ssize_t from;
     ssize_t to;
     ssize_t start; // starting position in concatenated file
     int fd;
+    int dirty;
 };
 
 static struct {
@@ -53,57 +59,193 @@ static struct {
     time_t mtime;
     time_t ctime;
 } times;
-    
-#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 );
+
+/**
+ * Overlay
+ */
+static struct {
+    struct Source source;
+    struct Region *table;
+    size_t count;
+    size_t limit;
+} overlay;
+
+static void usage();
+
+#define FRAG(m) (overlay.table+m)
+#define BEG(m) (FRAG(m)->pos)
+#define END(m) (FRAG(m)->pos + FRAG(m)->size)
+
+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 BEG( m ) < pos? m : -1;
+	}
+	if ( BEG( m ) <= 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;
+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;
+    }
+}
+
+static void overlay_save_table(size_t lo,size_t hi) {
+    char *p = (char *) FRAG(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;
+    }
+}
+
+static void overlay_insert(size_t p,off_t pos,size_t size) {
+    size_t bytes;
+    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( FRAG( p+1 ), FRAG( p ), bytes );
     }
+    FRAG( p )->pos = pos;
+    FRAG( p )->size = size;
+    overlay_save_count();
+}
+
+static void overlay_delete(size_t p) {
+    if ( p < --overlay.count ) {
+	size_t size = ( overlay.count - p ) * sizeof( struct Region );
+	memmove( FRAG(p), FRAG(p+1), size );
+    }
+    overlay_save_count();
+}
+
+static void overlay_mark(off_t pos,size_t size) {
 #if DEBUG
-    fprintf( stderr, "m=%d s=%d\n", m, s );
+    fprintf( stderr, "overlay_mark( %ld, %ld )\n", pos, size );
 #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; 
+    int deleted = 0;
+    ssize_t q;
+    ssize_t p = overlay_prior_fragment( pos );
+    // p is the nearest region below pos (or -1)
+    if ( p >= 0 && pos <= END(p) ) {
+	// p overlaps mark region
+	if ( END(p) >= pos + size ) {
+#if DEBUG
+	fprintf( stderr, "overlay size 1( %ld )\n", FRAG(p)->size );
+#endif
+	    return; // new mark within existing.
+	}
+	// new mark region extends existing
+	FRAG(p)->size = pos + size - BEG(p);
+	q = p+1;
+	while ( q < overlay.count && BEG(q) <= END(p) ) {
+	    if ( END(q) > END(p) ) {
+		FRAG(p)->size = END(q) - BEG(p);
+	    }
+	    overlay_delete( q );
+	    deleted++;
+	}
+	overlay_save_table( p, deleted? overlay.count : q );
+#if DEBUG
+	fprintf( stderr, "overlay size 2( %ld ) deleted %d\n",
+		 FRAG(p)->size, deleted );
+#endif
+	return;
     }
-    p->from = ( s < 0 )? 0 : atol( in+s+1 );
-    if ( p->from < 0 ) {
-	p->from = 0;
+    // The region p does not expand into new mark region
+    p++; // subsequent region
+    if ( p >= overlay.count || BEG(p) > pos + size ) {
+	// New mark is separate region at p
+	overlay_insert( p, pos, size);
+#if DEBUG
+	fprintf( stderr, "overlay size 4( %ld )\n", FRAG(p)->size );
+#endif
+	overlay_save_table( p, overlay.count );
+	return;
     }
+    // New marks start before and overlap with the region
+    if ( BEG(p) + FRAG(p)->size < pos + size ) {
+	FRAG(p)->size = size; // new mark covers old region
+    } else {
+	FRAG(p)->size += BEG(p) - pos;
+    }
+    BEG(p) = pos;
+    q = p+1;
+    while ( q < overlay.count && BEG(q) <= END(p) ) {
+	if ( END(q) > END(p) ) {
+	    FRAG(p)->size = END(q) - BEG(p);
+	}
+	overlay_delete( q );
+	deleted++;
+    }
+    overlay_save_table( p, deleted? overlay.count : q );
 #if DEBUG
-    fprintf( stderr, "p->from=%ld\n", p->from );
+    fprintf( stderr, "overlay size 4( %ld ) deleted %d\n",
+	     FRAG(p)->size, 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) {
@@ -115,22 +257,75 @@ static int setup_sources(char **argv,int i,int 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;
-	}
-	p->start = sources.size;
-	sources.size += p->to - p->from;
+	// 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 ) {
-	    fprintf( stderr, "** %s opened read-only\n", p->filename );
+	    rdonly = 1;
 	    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
@@ -186,17 +381,71 @@ static int find_source(off_t offset) {
     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;
 }
 
+static int overlay_merge(char *buf,off_t off,size_t size) {
+#if DEBUG
+    fprintf( stderr, "merge %ld %ld\n", off, size );
+#endif
+    // Find nearest overlay data before or at off
+    ssize_t p = overlay_prior_fragment( off );
+    if ( p < 0 ) {
+	p = 0;
+    }
+    for ( ; p < overlay.count && BEG(p) < off+size; p++ ) {
+	if ( END(p) < off ) {
+	    continue;
+	}
+	size_t delta = FRAG(p)->size;
+	if ( BEG(p) < off ) {
+	    delta -= off - BEG(p);
+	} else {
+	    size_t skip = BEG(p) - off;
+	    off += skip;
+	    size -= skip;
+	    buf += skip;
+	}
+	if ( delta > size ) {
+	    delta = size;
+	}
+	lseek( overlay.source.fd, off, SEEK_SET );
+	while ( delta > 0 ) {
+	    size_t n = read( overlay.source.fd, buf, delta );
+	    off += n;
+	    size -= n;
+	    delta -= n;
+	    buf += n;
+	}
+    }
+#if DEBUG
+    fprintf( stderr, "merged\n" );
+#endif
+    return 0;
+}
+
 // 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)
@@ -210,14 +459,14 @@ static int fusefile_read(const char *path, char *buf, size_t size,
 #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 ( off == sources.size )? 0 : -ENOENT;
+	    return ( off == sources.size )? rr : -ENOENT;
 	}
 	if ( sources.array[i].fd < 0 ) {
 	    return -ENOENT;
@@ -225,21 +474,30 @@ 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 );
@@ -248,14 +506,69 @@ 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, 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;
 }
 
+/**
+ * Poll for IO readiness.
+ */
+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.pos, f.size );
+#endif
+	overlay_mark( f.pos, f.size );
+    }
+}
+
 /**
  * Write a full block of data over the sources at the offset
  */
@@ -263,14 +576,18 @@ 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, size ); // Mark region as written
+    }
     while ( size > 0 ) {
 	int index = find_source( off ); // index of source file
 	if ( index < 0 ) {
 	    return -EIO; // past EOF
 	}
-	struct Source *source = &sources.array[ index ];
-	off_t from = off - source->start;
-	off_t max = source->to - source->from - from;
+	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;
 	}
@@ -286,6 +603,10 @@ static int write_block(off_t off,const char *buf,size_t size) {
 	    size -= n;
 	    off += n;
 	}
+	if ( source->dirty++ >= 1000 ) {
+	    fsync( source->fd );
+	    source->dirty = 0;
+	}
     }
     return 0;
 }
@@ -349,6 +670,20 @@ 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 );
@@ -356,6 +691,7 @@ static int fusefile_flush(const char *path, struct fuse_file_info *info) {
     if ( strcmp( path, "/" ) != 0 ) {
 	return -ENOENT;
     }
+    fsync_all_dirty();
     return 0;
 }
 
@@ -376,6 +712,7 @@ 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;
 }
 
@@ -392,11 +729,25 @@ 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,
@@ -406,13 +757,13 @@ 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 );
@@ -420,28 +771,31 @@ 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) {
+    // note: (*argv)[ argc ] is the mount point argument
     char *OURS[] = {
 	"-odefault_permissions",
-	(*argv)[ --argc ]  // note: (*argv)[ argc-1 ] = the mount point
+	(*argv)[ argc ]
     };
 #define OURSN ( sizeof( OURS ) / sizeof( char* ) )
-    int N = argc + OURSN; // new argv-tobe size, excluding null
-    char **out = malloc( ( N + 1 ) * 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 ] );
+	//fprintf( stderr, " %s", out[ i ] );
     }
     for ( i = 0; i < OURSN; i++ ) {
 	out[ argc + i ] = OURS[i];
-	fprintf( stderr, " %s", out[ i ] );
+	//fprintf( stderr, " %s", out[ i ] );
     }
     out[ N ] = 0;
-    fprintf( stderr, "\n" );
+    //fprintf( stderr, "\n" );
     (*argv) = out;
-    return N;
+    return N; // Don't include the terminating null pointer
 }
 
 /**
@@ -466,11 +820,24 @@ 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;
+    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();
+	}
+    }
     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 ) {
@@ -492,6 +859,16 @@ 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 ) ) {