vmod_debug: Add a utility function to log the origin of strands

This is implemented as a logging function to not use workspace itself.

bin/varnishtest/tests/v00058.vtc

-varnishtest "Test VRT STRANDS functions"
+varnishtest "Test VRT STRANDS related functions"
 
 varnish v1 -arg "-i foobar" -vcl {
 	import debug;
@@ -85,6 +85,11 @@ varnish v1 -arg "-i foobar" -vcl {
 			    req.url + "<-->" + req.url
 			)
 		    );
+		debug.log_strands("return_strands",
+			debug.return_strands(
+			    req.url + "<-->" + req.url
+			)
+		);
 	}
 } -start
 

vmod/vmod_debug.c

@@ -1288,3 +1288,77 @@ xyzzy_use_reembarking_http1(VRT_CTX)
 {
 	debug_transport_reembarking_http1_use(ctx);
 }
+
+static int
+in_oc(struct worker *wrk, struct objcore *oc, const char *p)
+{
+	const char *hdrs;
+	ssize_t len = 0;
+
+	if (oc == NULL)
+		return (0);
+	hdrs = ObjGetAttr(wrk, oc, OA_HEADERS, &len);
+	if (hdrs == NULL)
+		return (0);
+	if (p < hdrs)
+		return (0);
+	if (p > hdrs + len)
+		return (0);
+	return (1);
+}
+
+static const char *
+ptr_where(VRT_CTX, const char *p)
+{
+	struct ws *ws = ctx->ws;
+	struct ws *aws;
+	struct worker *wrk;
+	struct objcore *oc, *stale_oc;
+
+	if (ctx->req != NULL) {
+		wrk = ctx->req->wrk;
+		oc = ctx->req->objcore;
+		stale_oc = ctx->req->stale_oc;
+	}
+	else if (ctx->bo != NULL) {
+		wrk = ctx->bo->wrk;
+		oc = ctx->bo->fetch_objcore;
+		stale_oc = ctx->bo->stale_oc;
+	}
+	else
+		WRONG("ctx");
+
+	AN(wrk);
+	aws = ctx->req->wrk->aws;
+
+	if (WS_Allocated(ws, p, -1))
+		return ("ws");
+	if (WS_Allocated(aws, p, -1))
+		return ("aws");
+	if (in_oc(wrk, oc, p))
+		return ("oc");
+	if (in_oc(wrk, stale_oc, p))
+		return ("stale_oc");
+	return ("?");
+}
+
+VCL_VOID
+xyzzy_log_strands(VRT_CTX, VCL_STRING prefix, VCL_STRANDS subject, VCL_INT nn)
+{
+	int i, n;
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	if (prefix == NULL)
+		prefix = "";
+	AN(subject);
+	if (nn > INT_MAX)
+		n = INT_MAX;
+	else
+		n = nn;
+
+	for (i = 0; i < subject->n; i++) {
+		const char *p = subject->p[i];
+		mylog(ctx->vsl, SLT_Debug, "%s[%d]: (%s) %p %.*s%s", prefix, i,
+		    ptr_where(ctx, p), p, n, p, strlen(p) > n ? "..." : "");
+	}
+}

vmod/vmod_debug.vcc

@@ -447,6 +447,25 @@ $Restrict vcl_deliver
 Switch to the reembarking http1 debug transport. Calling it from any other
 transport than http1 results in VCL failure.
 
+$Function VOID log_strands(STRING prefix, STRANDS subject, INT n=4)
+
+For each strands member, emit one ``Debug`` log line as formatted using
+``printf()`` with prefix as ``%s``, the strands member index as ``[%d]:``, the
+*origin* as ``(%s)``, addresses of each strands as ``%p`` and at most the first
+*n* characters of each strand, with truncations marked by "...".
+
+*origin* can be one of:
+
+* ``ws`` if from ``ctx->ws``
+* ``aws`` if from ``wrk->aws``
+* ``oc`` if from ``OA_HEADERS`` of ``req->objcore`` or ``bo->fetch_objcore``
+* ``stale_oc`` if from ``OA_HEADERS`` of ``req->stale_oc`` or bo->stale_oc``
+* ``?`` otherwise
+
+Example::
+
+	Debug		c prefix[0]: (ws) 0x7fe69ef80420 abcd...
+
 DEPRECATED
 ==========