[PATCH v3 2/2] usage: add warn_once() helper for repeated warnings

["Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com>]

From: Derrick Stolee <derrickstolee@github.com>

The previous change added a warning when valid_remote() detects
credentials in the URL. Since remotes are validated multiple times per
process, this causes multiple warnings to print.

To avoid these kinds of repeated, advisory warnings, create a new
warn_once() helper that behaves the same as warning(), but only after
formatting the output string and adding it to a strset. If that addition
signals that the string already exists in the strset, then do not print
the warning.

In the case of the credentials in a URL, the existing test demonstrates
this per-process limitation: 'git clone' runs 'git-remote-curl' as a
child process, giving two messages. This is an improvement over the
previous six messages.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
---
 git-compat-util.h |  1 +
 remote.c          |  2 +-
 t/t5601-clone.sh  |  5 ++++-
 usage.c           | 22 ++++++++++++++++++++++
 4 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/git-compat-util.h b/git-compat-util.h
index 58fd813bd01..776a5f660aa 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -526,6 +526,7 @@ int error(const char *err, ...) __attribute__((format (printf, 1, 2)));
 int error_errno(const char *err, ...) __attribute__((format (printf, 1, 2)));
 void warning(const char *err, ...) __attribute__((format (printf, 1, 2)));
 void warning_errno(const char *err, ...) __attribute__((format (printf, 1, 2)));
+void warn_once(const char *warn, ...) __attribute__((format (printf, 1, 2)));
 
 #ifndef NO_OPENSSL
 #ifdef APPLE_COMMON_CRYPTO
diff --git a/remote.c b/remote.c
index accf08bf51f..72fffd0d968 100644
--- a/remote.c
+++ b/remote.c
@@ -60,7 +60,7 @@ static void check_if_creds_in_url(const char *url)
 		      "<redacted>", 10);
 
 	if (!strcmp("warn", value))
-		warning(_("URL '%s' uses plaintext credentials"), redacted.buf);
+		warn_once(_("URL '%s' uses plaintext credentials"), redacted.buf);
 	if (!strcmp("die", value))
 		die(_("URL '%s' uses plaintext credentials"), redacted.buf);
 
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index cba3553b7c4..6ae3eec9eb6 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -75,7 +75,10 @@ test_expect_success 'clone warns or fails when using username:password' '
 	test_must_fail git -c fetch.credentialsInUrl=allow clone https://username:password@localhost attempt1 2>err &&
 	! grep "URL '\''https://username:<redacted>@localhost'\'' uses plaintext credentials" err &&
 	test_must_fail git -c fetch.credentialsInUrl=warn clone https://username:password@localhost attempt1 2>err &&
-	grep "warning: URL '\''https://username:<redacted>@localhost'\'' uses plaintext credentials" err &&
+	grep "warning: URL '\''https://username:<redacted>@localhost'\'' uses plaintext credentials" err >warnings &&
+	# The warning is printed twice, for the two processes:
+	# "git clone" and "git-remote-curl".
+	test_line_count = 2 warnings &&
 	test_must_fail git -c fetch.credentialsInUrl=die clone https://username:password@localhost attempt2 2>err &&
 	grep "fatal: URL '\''https://username:<redacted>@localhost'\'' uses plaintext credentials" err
 '
diff --git a/usage.c b/usage.c
index b738dd178b3..242633c5f8d 100644
--- a/usage.c
+++ b/usage.c
@@ -5,6 +5,7 @@
  */
 #include "git-compat-util.h"
 #include "cache.h"
+#include "strmap.h"
 
 static void vreportf(const char *prefix, const char *err, va_list params)
 {
@@ -287,6 +288,27 @@ void warning(const char *warn, ...)
 	va_end(params);
 }
 
+static struct strset prev_warnings = STRSET_INIT;
+
+void warn_once(const char *warn, ...)
+{
+	char buf[1024];
+	va_list params;
+	va_start(params, warn);
+
+	if (vsnprintf(buf, sizeof(buf), warn, params) >= 0) {
+		if (!strset_add(&prev_warnings, buf)) {
+			va_end(params);
+			return;
+		}
+	}
+	va_end(params);
+
+	va_start(params, warn);
+	warn_routine(warn, params);
+	va_end(params);
+}
+
 /* Only set this, ever, from t/helper/, when verifying that bugs are caught. */
 int BUG_exit_code;
 
-- 
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