use http header 'Cache-Control: no-cache' for soft failure

This is standard header...

4 files changed, 66 insertions, 50 deletions

diff --git a/README.md b/README.md
index ff1091a..44c4182 100644
--- a/README.md
+++ b/README.md
@@ -99,16 +99,17 @@ This is the simplest workaround - just disable the server error limit.
 
 We can agree this is not to be desired - in general the feature is reasonable.
 
-### Support http header to indicate an expected failure
+### Support http header to indicate a soft failure
 
 This solution is simple, yet powerful:
-[Support http header to indicate an expected failure](patches/0001-support-http-header-to-indicate-an-expected-failure.patch)
+[Support http header 'Cache-Control: no-cache' for soft failure](patches/0001-support-http-header-Cache-Control-no-cache-for-soft-failure.patch)
 
-By setting an extra HTTP header `X-Pacman-Expected-Failure` the server can
-indicate that the failure is expected. The next server is tried without
-error message and without increasing the server's error count.
+By setting the HTTP header `Cache-Control: no-cache` when returning with
+the status code `404` (not found) the server can indicate that this is a
+soft failure. No error message is shown, and server's error count is
+not increased.
 
-Sadly upstream denied. 😢
+Sadly upstream denied. 😢 (At least an earlier version of the patch...)
 
 ### Implement CacheServer
 
diff --git a/pacredir.c b/pacredir.c
index 0366888..c5cc23f 100644
--- a/pacredir.c
+++ b/pacredir.c
@@ -578,7 +578,7 @@ response:
 		page = malloc(strlen(PAGE404) + strlen(basename) + 1);
 		sprintf(page, PAGE404, basename);
 		response = MHD_create_response_from_buffer(strlen(page), (void*) page, MHD_RESPMEM_MUST_FREE);
-		ret = MHD_add_response_header(response, "X-Pacman-Expected-Failure", "true");
+		ret = MHD_add_response_header(response, "Cache-Control", "no-cache");
 		ret = MHD_queue_response(connection, MHD_HTTP_NOT_FOUND, response);
 	}
 
diff --git a/patches/0001-support-http-header-Cache-Control-no-cache-for-soft-failure.patch b/patches/0001-support-http-header-Cache-Control-no-cache-for-soft-failure.patch
new file mode 100644
index 0000000..c81432a
--- /dev/null
+++ b/patches/0001-support-http-header-Cache-Control-no-cache-for-soft-failure.patch
@@ -0,0 +1,58 @@
+From 1288a202b61cf01fbff8a8bd0071e57383aa954e Mon Sep 17 00:00:00 2001
+From: Christian Hesse <mail@eworm.de>
+Date: Fri, 21 May 2021 09:52:34 +0200
+Subject: [PATCH 1/1] support http header 'Cache-Control: no-cache' for soft
+ failure
+
+By setting the HTTP header 'Cache-Control: no-cache' when returning with
+the status code 404 (not found) the server can indicate that this is a
+soft failure. No error message is shown, and server's error count is
+not increased.
+
+This can be used by servers that are not expected to be complete, for
+example when serving a local cache [0]. In nginx this can be achived by
+adding a single directive in location block:
+
+    add_header Cache-Control "no-cache";
+
+Also this is a perfect match for pacredir [1].
+
+[0] https://wiki.archlinux.org/title/Pacman/Tips_and_tricks#Network_shared_pacman_cache
+[1] https://git.eworm.de/cgit/pacredir/about/
+
+Signed-off-by: Christian Hesse <mail@eworm.de>
+---
+ lib/libalpm/dload.c | 11 +++++++++++
+ 1 file changed, 11 insertions(+)
+
+diff --git a/lib/libalpm/dload.c b/lib/libalpm/dload.c
+index 4fa17b35..23034584 100644
+--- a/lib/libalpm/dload.c
++++ b/lib/libalpm/dload.c
+@@ -274,8 +274,10 @@ static size_t dload_parseheader_cb(void *ptr, size_t size, size_t nmemb, void *u
+ {
+ 	size_t realsize = size * nmemb;
+ 	const char *fptr, *endptr = NULL;
++	const char * const cc_header = "Cache-Control:";
+ 	const char * const cd_header = "Content-Disposition:";
+ 	const char * const fn_key = "filename=";
++	const char * const nc_key = "no-cache";
+ 	struct dload_payload *payload = (struct dload_payload *)user;
+ 	long respcode;
+ 
+@@ -302,6 +304,15 @@ static size_t dload_parseheader_cb(void *ptr, size_t size, size_t nmemb, void *u
+ 		}
+ 	}
+ 
++	/* By setting the HTTP header 'Cache-Control: no-cache' the server can indicate
++	   that this is a soft failure which should not be cached. No error message is
++	   shown, and server's error count is not increased. */
++	if(_alpm_raw_ncmp(cc_header, ptr, strlen(cc_header)) == 0) {
++		if(strstr(ptr, nc_key)) {
++			payload->errors_ok = 1;
++		}
++	}
++
+ 	curl_easy_getinfo(payload->curl, CURLINFO_RESPONSE_CODE, &respcode);
+ 	if(payload->respcode != respcode) {
+ 		payload->respcode = respcode;
diff --git a/patches/0001-support-http-header-to-indicate-an-expected-failure.patch b/patches/0001-support-http-header-to-indicate-an-expected-failure.patch
deleted file mode 100644
index 6bc977a..0000000
--- a/patches/0001-support-http-header-to-indicate-an-expected-failure.patch
+++ /dev/null
@@ -1,43 +0,0 @@
-From b50863ad56c3c972e4bfbc9108523fee267d63fa Mon Sep 17 00:00:00 2001
-From: Christian Hesse <mail@eworm.de>
-Date: Fri, 21 May 2021 09:52:34 +0200
-Subject: [PATCH] support http header to indicate an expected failure
-
-By setting an extra HTTP header 'X-Pacman-Expected-Failure' the server can
-indicate that the failure is expected. The next server is tried without
-error message and without increasing the server's error count.
-
-This can be used by servers that are not expected to be complete, for
-example when serving a local cache.
-
-Signed-off-by: Christian Hesse <mail@eworm.de>
----
- lib/libalpm/dload.c | 8 ++++++++
- 1 file changed, 8 insertions(+)
-
-diff --git a/lib/libalpm/dload.c b/lib/libalpm/dload.c
-index 7c27c3ea..d39125ca 100644
---- a/lib/libalpm/dload.c
-+++ b/lib/libalpm/dload.c
-@@ -277,6 +277,7 @@ static size_t dload_parseheader_cb(void *ptr, size_t size, size_t nmemb, void *u
- 	const char *fptr, *endptr = NULL;
- 	const char * const cd_header = "Content-Disposition:";
- 	const char * const fn_key = "filename=";
-+	const char * const xpef_header = "X-Pacman-Expected-Failure:";
- 	struct dload_payload *payload = (struct dload_payload *)user;
- 	long respcode;
- 
-@@ -303,6 +304,13 @@ static size_t dload_parseheader_cb(void *ptr, size_t size, size_t nmemb, void *u
- 		}
- 	}
- 
-+	/* By setting an extra HTTP header 'X-Pacman-Expected-Failure' the server can
-+	   indicate that the failure is expected. The next server is tried without
-+	   error message and without increasing the server's error count. */
-+	if(_alpm_raw_ncmp(xpef_header, ptr, strlen(xpef_header)) == 0) {
-+		payload->errors_ok = 1;
-+	}
-+
- 	curl_easy_getinfo(payload->curl, CURLINFO_RESPONSE_CODE, &respcode);
- 	if(payload->respcode != respcode) {
- 		payload->respcode = respcode;