avoid copying for IDENTITY decode if possible

Patch from Nils

src/vmod_blobcode.c

@@ -325,9 +325,70 @@ vmod_blob__fini(struct vmod_blobcode_blob **blobp)
 
 /* Functions */
 
+static inline const char *
+find_nonempty_va(const char *restrict *p, va_list ap) {
+	const char *q;
+
+	/* find first non-empty vararg */
+	for (; *p == vrt_magic_string_end || *p == NULL || **p == '\0';
+	     *p = va_arg(ap, const char *))
+		if (*p == vrt_magic_string_end)
+			return (vrt_magic_string_end);
+
+	/* find next non-empty vararg */
+	for (q = va_arg(ap, const char *);
+	     q != vrt_magic_string_end && (q == NULL || *q == '\0');
+	     q = va_arg(ap, const char *))
+		;
+
+	return (q);
+}
+
+/*
+ * special case: we can avoid copying for identity decode if we need to
+ * deal with a single vararg only - in which case we just have the blob
+ * point to the input string
+ */
+static VCL_BLOB
+decode_id_inplace(struct vmod_priv *b, VCL_INT n, const char *restrict p,
+		  va_list ap) {
+	const char *q;
+	int l;
+
+	if (n == 0)
+		return null_blob;
+
+	q = find_nonempty_va(&p, ap);
+
+	if (p == vrt_magic_string_end)
+		return null_blob;
+
+	if (q == vrt_magic_string_end) {
+		/* can use in-place decode */
+		l = strlen(p);
+		if (n > 0 && n < l)
+			l = n;
+		b->priv = (char *)p;
+		b->len = l;
+		b->free = NULL;
+		return (b);
+	}
+
+	if (n == -1)
+		return NULL;
+	l = strlen(p);
+	if (n > l)
+		return NULL;
+	b->priv = (char *)p;
+	b->len = n;
+	b->free = NULL;
+
+	return (b);
+}
+
 static VCL_BLOB
-decode(VRT_CTX, VCL_INT n, VCL_ENUM decs, const char *restrict const p,
-       va_list ap) {
+decode(VRT_CTX, VCL_INT n, VCL_ENUM decs,
+       const char *restrict const p, va_list ap) {
 	enum encoding dec = parse_encoding(decs);
 	struct wb_s wb;
 	struct vmod_priv *b;
@@ -348,6 +409,18 @@ decode(VRT_CTX, VCL_INT n, VCL_ENUM decs, const char *restrict const p,
 		ERRNOMEM(ctx, "cannot decode");
 		return NULL;
 	}
+
+	if (dec == IDENTITY) {
+		va_list aq;
+		va_copy(aq, ap);
+		const struct vmod_priv *bb = decode_id_inplace(b, n, p, aq);
+		va_end(aq);
+		if (bb == null_blob)
+			WS_Reset(ctx->ws, snap);
+		if (bb == b)
+			return bb;
+	}
+
 	if (wb_create(ctx->ws, &wb) == NULL) {
 		WS_Reset(ctx->ws, snap);
 		ERRNOMEM(ctx, "cannot decode");
@@ -401,9 +474,8 @@ vmod_decode_n(VRT_CTX, VCL_INT n, VCL_ENUM decs, const char *p, ...) {
 	return (r);
 }
 
-VCL_STRING __match_proto__(td_blobcode_encode)
-vmod_encode(VRT_CTX, VCL_ENUM encs, VCL_BLOB b) {
-	enum encoding enc = parse_encoding(encs);
+static VCL_STRING
+encode(VRT_CTX, enum encoding enc, VCL_BLOB b) {
 	struct wb_s wb;
 	ssize_t len;
 
@@ -439,6 +511,12 @@ vmod_encode(VRT_CTX, VCL_ENUM encs, VCL_BLOB b) {
 	return wb_finish(&wb, NULL);
 }
 
+VCL_STRING __match_proto__(td_blobcode_encode)
+vmod_encode(VRT_CTX, VCL_ENUM encs, VCL_BLOB b) {
+	enum encoding enc = parse_encoding(encs);
+	return encode(ctx, enc, b);
+}
+
 static VCL_STRING
 transcode(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
 	  const char *restrict const p, va_list ap) {
@@ -458,6 +536,17 @@ transcode(VRT_CTX, VCL_INT n, VCL_ENUM decs, VCL_ENUM encs,
 	AENC(dec);
 	AENC(enc);
 
+	if (dec == IDENTITY) {
+		struct vmod_priv b;
+		va_copy(aq, ap);
+		const struct vmod_priv *bb = decode_id_inplace(&b, n, p, aq);
+		va_end(aq);
+		if (bb != NULL) {
+			VCL_STRING r = encode(ctx, enc, bb);
+			return (r);
+		}
+	}
+
 	/*
 	 * Allocate space for the decoded blob on the stack
 	 * ignoring the limitation imposed by n