diff --git a/ChangeLog b/ChangeLog
index 298c9fab..49a599cc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -19,6 +19,7 @@
 - better gifload behaviour for DISPOSAL_UNSPECIFIED [DarthSim]
 - ban ppm max_value < 0 
 - add fuzz corpus to dist
+- detect read errors correctly in source_sniff
 
 20/6/19 started 8.9.1
 - don't use the new source loaders for new_from_file or new_from_buffer, it
diff --git a/autogen.sh b/autogen.sh
index af5c3d1b..607b795b 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -61,4 +61,6 @@ autoheader
 $LIBTOOLIZE --copy --force --automake
 automake --add-missing --copy
 
-./configure $*
+if test -z "$NOCONFIGURE"; then
+  ./configure $*
+fi
diff --git a/libvips/arithmetic/boolean.c b/libvips/arithmetic/boolean.c
index c4fcf18d..ad978fe1 100644
--- a/libvips/arithmetic/boolean.c
+++ b/libvips/arithmetic/boolean.c
@@ -562,7 +562,7 @@ vips_boolean_const_init( VipsBooleanConst *boolean_const )
 
 static int
 vips_boolean_constv( VipsImage *in, VipsImage **out, 
-	VipsOperationBoolean operation, double *c, int n, va_list ap )
+	VipsOperationBoolean operation, const double *c, int n, va_list ap )
 {
 	VipsArea *area_c;
 	double *array; 
@@ -609,7 +609,7 @@ vips_boolean_constv( VipsImage *in, VipsImage **out,
  */
 int
 vips_boolean_const( VipsImage *in, VipsImage **out, 
-	VipsOperationBoolean boolean, double *c, int n, ... )
+	VipsOperationBoolean boolean, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -637,7 +637,8 @@ vips_boolean_const( VipsImage *in, VipsImage **out,
  * Returns: 0 on success, -1 on error
  */
 int
-vips_andimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_andimage_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -666,7 +667,8 @@ vips_andimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_orimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_orimage_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -695,7 +697,8 @@ vips_orimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_eorimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_eorimage_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -724,7 +727,7 @@ vips_eorimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_lshift_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_lshift_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -753,7 +756,7 @@ vips_lshift_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_rshift_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_rshift_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
diff --git a/libvips/arithmetic/linear.c b/libvips/arithmetic/linear.c
index 4a376a55..ebf4fb33 100644
--- a/libvips/arithmetic/linear.c
+++ b/libvips/arithmetic/linear.c
@@ -453,7 +453,7 @@ vips_linear_init( VipsLinear *linear )
 
 static int
 vips_linearv( VipsImage *in, VipsImage **out, 
-	double *a, double *b, int n, va_list ap )
+	const double *a, const double *b, int n, va_list ap )
 {
 	VipsArea *area_a;
 	VipsArea *area_b;
@@ -500,7 +500,8 @@ vips_linearv( VipsImage *in, VipsImage **out,
  * Returns: 0 on success, -1 on error
  */
 int
-vips_linear( VipsImage *in, VipsImage **out, double *a, double *b, int n, ... )
+vips_linear( VipsImage *in, VipsImage **out, 
+	const double *a, const double *b, int n, ... )
 {
 	va_list ap;
 	int result;
diff --git a/libvips/arithmetic/math2.c b/libvips/arithmetic/math2.c
index cbcd2b99..dd3e5323 100644
--- a/libvips/arithmetic/math2.c
+++ b/libvips/arithmetic/math2.c
@@ -420,7 +420,7 @@ vips_math2_const_init( VipsMath2Const *math2_const )
 
 static int
 vips_math2_constv( VipsImage *in, VipsImage **out, 
-	VipsOperationMath2 math2, double *c, int n, va_list ap )
+	VipsOperationMath2 math2, const double *c, int n, va_list ap )
 {
 	VipsArea *area_c;
 	double *array; 
@@ -470,7 +470,7 @@ vips_math2_constv( VipsImage *in, VipsImage **out,
  */
 int
 vips_math2_const( VipsImage *in, VipsImage **out, 
-	VipsOperationMath2 math2, double *c, int n, ... )
+	VipsOperationMath2 math2, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -496,7 +496,7 @@ vips_math2_const( VipsImage *in, VipsImage **out,
  * Returns: 0 on success, -1 on error
  */
 int
-vips_pow_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_pow_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -523,7 +523,7 @@ vips_pow_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_wop_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_wop_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
diff --git a/libvips/arithmetic/relational.c b/libvips/arithmetic/relational.c
index f2f8f817..d740ea5c 100644
--- a/libvips/arithmetic/relational.c
+++ b/libvips/arithmetic/relational.c
@@ -598,7 +598,7 @@ vips_relational_const_init( VipsRelationalConst *relational_const )
 
 static int
 vips_relational_constv( VipsImage *in, VipsImage **out, 
-	VipsOperationRelational relational, double *c, int n, va_list ap )
+	VipsOperationRelational relational, const double *c, int n, va_list ap )
 {
 	VipsArea *area_c;
 	double *array; 
@@ -645,7 +645,7 @@ vips_relational_constv( VipsImage *in, VipsImage **out,
  */
 int
 vips_relational_const( VipsImage *in, VipsImage **out, 
-	VipsOperationRelational relational, double *c, int n, ... )
+	VipsOperationRelational relational, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -671,7 +671,7 @@ vips_relational_const( VipsImage *in, VipsImage **out,
  * Returns: 0 on success, -1 on error
  */
 int
-vips_equal_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_equal_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -698,7 +698,8 @@ vips_equal_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_notequal_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_notequal_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -725,7 +726,7 @@ vips_notequal_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_less_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_less_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -752,7 +753,7 @@ vips_less_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_lesseq_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_lesseq_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -779,7 +780,7 @@ vips_lesseq_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_more_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_more_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
@@ -806,7 +807,7 @@ vips_more_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
  * Returns: 0 on success, -1 on error
  */
 int
-vips_moreeq_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_moreeq_const( VipsImage *in, VipsImage **out, const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
diff --git a/libvips/arithmetic/remainder.c b/libvips/arithmetic/remainder.c
index 75df8f4d..3c994345 100644
--- a/libvips/arithmetic/remainder.c
+++ b/libvips/arithmetic/remainder.c
@@ -333,7 +333,7 @@ vips_remainder_const_init( VipsRemainderConst *remainder_const )
 
 static int
 vips_remainder_constv( VipsImage *in, VipsImage **out, 
-	double *c, int n, va_list ap )
+	const double *c, int n, va_list ap )
 {
 	VipsArea *area_c;
 	double *array; 
@@ -379,7 +379,8 @@ vips_remainder_constv( VipsImage *in, VipsImage **out,
  * Returns: 0 on success, -1 on error
  */
 int
-vips_remainder_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+vips_remainder_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 {
 	va_list ap;
 	int result;
diff --git a/libvips/foreign/foreign.c b/libvips/foreign/foreign.c
index f85e6a47..a0669d55 100644
--- a/libvips/foreign/foreign.c
+++ b/libvips/foreign/foreign.c
@@ -535,7 +535,7 @@ vips_foreign_find_load( const char *name )
 	 */
 	if( !vips_existsf( "%s", filename ) ) {
 		vips_error( "VipsForeignLoad", 
-			_( "file \"%s\" not found" ), name );
+			_( "file \"%s\" not readable" ), name );
 		return( NULL );
 	}
 
diff --git a/libvips/include/vips/arithmetic.h b/libvips/include/vips/arithmetic.h
index ce95399a..3c9c7aa9 100644
--- a/libvips/include/vips/arithmetic.h
+++ b/libvips/include/vips/arithmetic.h
@@ -185,14 +185,14 @@ int vips_multiply( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 int vips_divide( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 	__attribute__((sentinel));
 int vips_linear( VipsImage *in, VipsImage **out, 
-	double *a, double *b, int n, ... )
+	const double *a, const double *b, int n, ... )
 	__attribute__((sentinel));
 int vips_linear1( VipsImage *in, VipsImage **out, double a, double b, ... )
 	__attribute__((sentinel));
 int vips_remainder( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 	__attribute__((sentinel));
 int vips_remainder_const( VipsImage *in, VipsImage **out, 
-	double *c, int n, ... )
+	const double *c, int n, ... )
 	__attribute__((sentinel));
 int vips_remainder_const1( VipsImage *in, VipsImage **out, 
 	double c, ... )
@@ -279,19 +279,25 @@ int vips_more( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 int vips_moreeq( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 	__attribute__((sentinel));
 int vips_relational_const( VipsImage *in, VipsImage **out, 
-	VipsOperationRelational relational, double *c, int n, ... )
+	VipsOperationRelational relational, const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_equal_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_equal_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_notequal_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_notequal_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_less_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_less_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_lesseq_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_lesseq_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_more_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_more_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_moreeq_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_moreeq_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
 int vips_relational_const1( VipsImage *in, VipsImage **out, 
 	VipsOperationRelational relational, double c, ... )
@@ -324,17 +330,22 @@ int vips_rshift( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 	__attribute__((sentinel));
 
 int vips_boolean_const( VipsImage *in, VipsImage **out, 
-	VipsOperationBoolean boolean, double *c, int n, ... )
+	VipsOperationBoolean boolean, const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_andimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_andimage_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_orimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_orimage_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_eorimage_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_eorimage_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_lshift_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_lshift_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_rshift_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_rshift_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
 int vips_boolean_const1( VipsImage *in, VipsImage **out, 
 	VipsOperationBoolean boolean, double c, ... )
@@ -358,11 +369,13 @@ int vips_pow( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 int vips_wop( VipsImage *left, VipsImage *right, VipsImage **out, ... )
 	__attribute__((sentinel));
 int vips_math2_const( VipsImage *in, VipsImage **out, 
-	VipsOperationMath2 math2, double *c, int n, ... )
+	VipsOperationMath2 math2, const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_pow_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_pow_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
-int vips_wop_const( VipsImage *in, VipsImage **out, double *c, int n, ... )
+int vips_wop_const( VipsImage *in, VipsImage **out, 
+	const double *c, int n, ... )
 	__attribute__((sentinel));
 int vips_math2_const1( VipsImage *in, VipsImage **out, 
 	VipsOperationMath2 math2, double c, ... )
diff --git a/libvips/include/vips/connection.h b/libvips/include/vips/connection.h
index d12a33a6..35132733 100644
--- a/libvips/include/vips/connection.h
+++ b/libvips/include/vips/connection.h
@@ -222,7 +222,7 @@ const void *vips_source_map( VipsSource *source, size_t *length );
 VipsBlob *vips_source_map_blob( VipsSource *source );
 gint64 vips_source_seek( VipsSource *source, gint64 offset, int whence );
 int vips_source_rewind( VipsSource *source );
-size_t vips_source_sniff_at_most( VipsSource *source, 
+gint64 vips_source_sniff_at_most( VipsSource *source, 
 	unsigned char **data, size_t length );
 unsigned char *vips_source_sniff( VipsSource *source, size_t length );
 gint64 vips_source_length( VipsSource *source ); 
diff --git a/libvips/iofuncs/memory.c b/libvips/iofuncs/memory.c
index db396e51..517cf301 100644
--- a/libvips/iofuncs/memory.c
+++ b/libvips/iofuncs/memory.c
@@ -358,7 +358,7 @@ vips_tracked_malloc( size_t size )
  * @flags: flags for open()
  * @...: open mode
  *
- * Exactly as open(2), but the number of files current open via
+ * Exactly as open(2), but the number of files currently open via
  * vips_tracked_open() is available via vips_tracked_get_files(). This is used
  * by the vips operation cache to drop cache when the number of files
  * available is low.
diff --git a/libvips/iofuncs/source.c b/libvips/iofuncs/source.c
index af482d11..4f07b0ce 100644
--- a/libvips/iofuncs/source.c
+++ b/libvips/iofuncs/source.c
@@ -1191,7 +1191,7 @@ vips_source_length( VipsSource *source )
 }
 
 /**
- * vips_source_peek: 
+ * vips_source_sniff_at_most: 
  * @source: peek this source
  * @data: return a pointer to the bytes read here
  * @length: max number of bytes to read
@@ -1203,7 +1203,7 @@ vips_source_length( VipsSource *source )
  *
  * Returns: number of bytes read, or -1 on error.
  */
-size_t
+gint64
 vips_source_sniff_at_most( VipsSource *source, 
 	unsigned char **data, size_t length )
 {
@@ -1257,12 +1257,14 @@ unsigned char *
 vips_source_sniff( VipsSource *source, size_t length )
 {
 	unsigned char *data;
-	size_t bytes_read;
+	gint64 bytes_read;
 
 	if( vips_source_test_features( source ) )
 		return( NULL );
 
 	bytes_read = vips_source_sniff_at_most( source, &data, length );
+	if( bytes_read == -1 )
+		return( NULL );
 	if( bytes_read < length )
 		return( NULL );
 
diff --git a/libvips/iofuncs/util.c b/libvips/iofuncs/util.c
index 99c67641..b749541d 100644
--- a/libvips/iofuncs/util.c
+++ b/libvips/iofuncs/util.c
@@ -634,6 +634,14 @@ vips__open( const char *filename, int flags, ... )
 	mode = va_arg( ap, int );
 	va_end( ap );
 
+	/* Various bad things happen if you accidentally open a directory as a
+	 * file.
+	 */
+	if( g_file_test( filename, G_FILE_TEST_IS_DIR ) ) {
+		errno = EISDIR;
+		return( -1 );
+	}
+
 	fd = g_open( filename, flags, mode );
 
 #ifdef OS_WIN32
@@ -1090,6 +1098,10 @@ vips__seek_no_error( int fd, gint64 pos, int whence )
 #ifdef OS_WIN32
 	new_pos = _lseeki64( fd, pos, whence );
 #else /*!OS_WIN32*/
+	/* On error, eg. opening a directory and seeking to the end, lseek() 
+	 * on linux seems to return 9223372036854775807 ((1 << 63) - 1)
+	 * rather than (off_t) -1 for reasons I don't understand. 
+	 */
 	new_pos = lseek( fd, pos, whence );
 #endif /*OS_WIN32*/
 
@@ -1145,29 +1157,26 @@ vips__ftruncate( int fd, gint64 pos )
 	return( 0 );
 }
 
-/* TRUE if file exists.
+/* TRUE if file exists and is not a directory.
  */
 gboolean
 vips_existsf( const char *name, ... )
 {
         va_list ap;
 	char *path; 
-        int result; 
+        gboolean result; 
 
         va_start( ap, name );
 	path = g_strdup_vprintf( name, ap ); 
         va_end( ap );
 
-        result = g_access( path, R_OK );
+	/* A regular (not a directory) file.
+	 */
+	result = g_file_test( path, G_FILE_TEST_IS_REGULAR );
 
 	g_free( path ); 
 
-	/* access() can fail for various reasons, especially under things 
-	 * like selinux. Only return FALSE if we are certain the file does not
-	 * exist.
-	 */
-	return( result == 0 || 
-		errno != ENOENT );
+	return( result ); 
 }
 
 #ifdef OS_WIN32