openslideload_source: flag operation as "nocache"

[kleisauke]

See commit cb58d7d for rationale.

History:
Commit 1214f94 moved this flag to the base class, and commit 53db48d later removed it, likely under the assumption that the _source class still had this flag set.

Targets the 8.17 branch.

[kleisauke]

Found by this (not yet committed) change in NetVips:

--- a/samples/NetVips.Samples/Samples/GenerateImageClass.cs
+++ b/samples/NetVips.Samples/Samples/GenerateImageClass.cs
@@ -156,7 +156,8 @@ private string GenerateFunction(string operationName, string indent = "
         bool mutable = false, IReadOnlyList<Introspect.Argument> outParameters = null)
     {
         using var op = Operation.NewFromName(operationName);
-        if ((op.GetFlags() & Enums.OperationFlags.DEPRECATED) != 0)
+        var flags = op.GetFlags();
+        if ((flags & Enums.OperationFlags.DEPRECATED) != 0)
         {
             throw new ArgumentException($"No such operator. Operator \"{operationName}\" is deprecated");
         }
@@ -172,8 +173,8 @@ private string GenerateFunction(string operationName, string indent = "
         // we are only interested in non-deprecated args
         var optionalInput = intro.OptionalInput
             .Where(arg => (arg.Value.Flags & Enums.ArgumentFlags.DEPRECATED) == 0)
-            // Drop "revalidate" option from source and buffer loaders as they are already uncached
-            .Where(arg => arg.Key != "revalidate" || (firstArgType != GValue.SourceType && firstArgType != GValue.BlobType))
+            // Drop "revalidate" option from buffer loaders and operations marked "nocache" as they are already uncached
+            .Where(arg => arg.Key != "revalidate" || ((flags & Enums.OperationFlags.NOCACHE) == 0 && firstArgType != GValue.BlobType))
             .Select(x => x.Value)
             .ToArray();
         var optionalOutput = intro.OptionalOutput

[jcupitt]

That's annoying. How about adding this to operation.c:

static void
vips_operation_summary_class(VipsObjectClass *object_class, VipsBuf *buf)
{
    VipsOperationClass *operation_class = VIPS_OPERATION_CLASS(object_class);

    VIPS_OBJECT_CLASS(vips_operation_parent_class)
        ->summary_class(object_class, buf);
 
    GType gtype = G_OBJECT_CLASS_TYPE(operation_class);
    if (!G_TYPE_IS_ABSTRACT(gtype) &&
        operation_class->flags & VIPS_OPERATION_NOCACHE)
        vips_buf_appendf(buf, ", nocache");
}

Now vips -l displays eg.:

$ vips -l | grep _source
          VipsForeignLoadCsvSource (csvload_source), load csv, nocache, priority=0, untrusted, is_a_source, get_flags, header, load
...

And we can test that every source loader is tagged correctly with:

$ vips -l | grep _source | grep -v nocache
          VipsForeignLoadOpenslideSource (openslideload_source), load source with OpenSlide, priority=100, untrusted, is_a_source, get_flags, get_flags_filename, header, load
$ 

(before this PR, of course hehe)

[kleisauke]

Good idea! Let's do that as a follow-up.