translate: generic plc not filled in after translation

If during translation a codec could not handle a given frame the translation
core would return NULL, thus not passing along the "missing" frame. Due to this
there was no frame to apply generic plc to, thus rendering it useless.

This patch makes it so the translation core produces an interpolated slin frame
in the cases where an attempt was made to translate to slin, but failed. This
interpolated frame is then passed along and can be used by the generic plc
algorithms to fill in the frame.

ASTERISK-27814 #close

Change-Id: I133d084da87adef913bf2ecc9c9240e3eaf4f40a

formats/format_sln.c

@@ -54,6 +54,12 @@ static struct ast_frame *generic_read(struct ast_filestream *s, int *whennext, u
 static int slinear_write(struct ast_filestream *fs, struct ast_frame *f)
 {
 	int res;
+
+	/* Don't try to write an interpolated frame */
+	if (f->datalen == 0) {
+		return 0;
+	}
+
 	if ((res = fwrite(f->data.ptr, 1, f->datalen, fs->f)) != f->datalen) {
 			ast_log(LOG_WARNING, "Bad write (%d/%d): %s\n", res, f->datalen, strerror(errno));
 			return -1;

main/translate.c

@@ -524,6 +524,34 @@ struct ast_trans_pvt *ast_translator_build_path(struct ast_format *dst, struct a
 	return head;
 }
 
+static struct ast_frame *generate_interpolated_slin(struct ast_trans_pvt *p, struct ast_frame *f)
+{
+	struct ast_frame res = { AST_FRAME_VOICE };
+
+	/*
+	 * If we've gotten here then we should have an interpolated frame that was not handled
+	 * by the translation codec. So create an interpolated frame in the appropriate format
+	 * that was going to be written. This frame might be handled later by other resources.
+	 * For instance, generic plc.
+	 *
+	 * Note, generic plc is currently only available for the format type 'slin' (8KHz only -
+	 * The generic plc code appears to have been based around that). Generic plc is filled
+	 * in later on frame write.
+	 */
+	if (!ast_opt_generic_plc || f->datalen != 0 ||
+		ast_format_cmp(p->explicit_dst, ast_format_slin) == AST_FORMAT_CMP_NOT_EQUAL) {
+		return NULL;
+	}
+
+	res.subclass.format = ast_format_cache_get_slin_by_rate(8000); /* ref bumped on dup */
+	res.samples = f->samples;
+	res.datalen = 0;
+	res.data.ptr = NULL;
+	res.offset = AST_FRIENDLY_OFFSET;
+
+	return ast_frdup(&res);
+}
+
 /*! \brief do the actual translation */
 struct ast_frame *ast_translate(struct ast_trans_pvt *path, struct ast_frame *f, int consume)
 {
@@ -586,6 +614,11 @@ struct ast_frame *ast_translate(struct ast_trans_pvt *path, struct ast_frame *f,
 		}
 		out = p->t->frameout(p);
 	}
+
+	if (!out) {
+		out = generate_interpolated_slin(path, f);
+	}
+
 	if (out) {
 		/* we have a frame, play with times */
 		if (!ast_tvzero(delivery)) {