[master] 764a86caf vcl: New bereq.retry_connect variable

[Walid Boudebouda]

commit 764a86cafb6f8f685d5ae9421f22fe157319c411
Author: Walid Boudebouda <walid.boudebouda at gmail.com>
Date:   Fri Nov 21 13:05:36 2025 +0100

    vcl: New bereq.retry_connect variable
    
    Controls whether Varnish will make a second attempt to connect to the
    backend if recycling an existing connection failed. This allows users
    to prevent undesired retries of potentially non-idempotent requests
    on backend connection failures, and provides a finer grained control.
    
    Fixes: #4413

diff --git a/bin/varnishd/cache/cache.h b/bin/varnishd/cache/cache.h
index fc5b9cf93..c86fa83f7 100644
--- a/bin/varnishd/cache/cache.h
+++ b/bin/varnishd/cache/cache.h
@@ -455,6 +455,7 @@ struct busyobj {
 #define BUSYOBJ_TMO(bo, pfx, tmo)					\
 	(isnan((bo)->tmo) ? cache_param->pfx##tmo : (bo)->tmo)
 
+extern const char *retry_disabled;
 
 /*--------------------------------------------------------------------*/
 
diff --git a/bin/varnishd/cache/cache_fetch.c b/bin/varnishd/cache/cache_fetch.c
index 0bfd34637..8f185ce4f 100644
--- a/bin/varnishd/cache/cache_fetch.c
+++ b/bin/varnishd/cache/cache_fetch.c
@@ -315,7 +315,7 @@ vbf_stp_retry(struct worker *wrk, struct busyobj *bo)
 
 	assert(bo->fetch_objcore->boc->state <= BOS_REQ_DONE);
 
-	if (bo->no_retry != NULL) {
+	if (bo->no_retry != NULL && bo->no_retry != retry_disabled) {
 		VSLb(bo->vsl, SLT_Error,
 		    "Retry not possible, %s", bo->no_retry);
 		return (F_STP_FAIL);
diff --git a/bin/varnishd/cache/cache_vrt_var.c b/bin/varnishd/cache/cache_vrt_var.c
index cc99f0153..a6d049764 100644
--- a/bin/varnishd/cache/cache_vrt_var.c
+++ b/bin/varnishd/cache/cache_vrt_var.c
@@ -48,6 +48,7 @@
 #define VRT_TMO(tmo) (isinf(tmo) ? VRT_DECIMAL_MAX : tmo)
 
 static char vrt_hostname[255] = "";
+const char *retry_disabled = "disabled from VCL";
 
 /*--------------------------------------------------------------------
  * VRT variables relating to first line of HTTP/1.1 req/resp
@@ -737,6 +738,28 @@ VRT_r_bereq_retries(VRT_CTX)
 	return (ctx->bo->retries);
 }
 
+VCL_VOID
+VRT_l_bereq_retry_connect(VRT_CTX, VCL_BOOL b)
+{
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	CHECK_OBJ_NOTNULL(ctx->bo, BUSYOBJ_MAGIC);
+
+	if (!b && ctx->bo->no_retry == NULL)
+		ctx->bo->no_retry = retry_disabled;
+	else if (b && ctx->bo->no_retry == retry_disabled)
+		ctx->bo->no_retry = NULL;
+}
+
+VCL_BOOL
+VRT_r_bereq_retry_connect(VRT_CTX)
+{
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	CHECK_OBJ_NOTNULL(ctx->bo, BUSYOBJ_MAGIC);
+	return (ctx->bo->no_retry == NULL);
+}
+
 /*--------------------------------------------------------------------*/
 
 VCL_STRING
diff --git a/doc/sphinx/reference/vcl_var.rst b/doc/sphinx/reference/vcl_var.rst
index 52cff63da..1c0d8e9ef 100644
--- a/doc/sphinx/reference/vcl_var.rst
+++ b/doc/sphinx/reference/vcl_var.rst
@@ -908,6 +908,27 @@ bereq.retries
 
 	A count of how many times this request has been retried.
 
+.. _bereq.retry_connect:
+
+bereq.retry_connect
+
+	Type: BOOL
+
+	Readable from: backend
+
+	Writable from: backend
+
+	Default: ``true``.
+	Controls whether Varnish will make a second attempt to connect to the
+	backend if a first connection reuse attempt failed. Setting to ``false``
+	means that no retries will be made. However, setting this to ``true``
+	does not guarantee that a retry will always be attempted, as there are
+	other factors involved in the decision (ex: a request body not being
+	cached).
+	Note that this only controls automatic retries due to connection
+	failures, and does not affect retries initiated from VCL using
+	``return(retry);``.
+
 
 .. _bereq.task_deadline: