prometheus_sysctl_exporter: don't abort on bad labels

We can probaby consider these kernel bugs, in which case asserting is
not the most helpful thing we can do.  Let's emit the necessary details
to stderr and exit non-zero to aid debugging these without completely
blocking the ability to export all of the well-formed metrics.

Reviewed by:	rew
Differential Revision:	https://reviews.freebsd.org/D57983

usr.sbin/prometheus_sysctl_exporter/prometheus_sysctl_exporter.8

@@ -20,7 +20,7 @@
 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
-.Dd October 7, 2021
+.Dd August 6, 2026
 .Dt PROMETHEUS_SYSCTL_EXPORTER 8
 .Os
 .Sh NAME
@@ -95,6 +95,11 @@ is to be a regular expression as described in
 The provided regular expression is tested against the Prometheus
 metric name.
 .El
+.Sh EXIT STATUS
+The
+.Nm
+utility exits 0 on success, and >0 if an error is encountered while processing
+metric labels.
 .Sh SEE ALSO
 .Xr cron 8 ,
 .Xr inetd 8 ,

usr.sbin/prometheus_sysctl_exporter/prometheus_sysctl_exporter.c

@@ -371,8 +371,14 @@ oid_get_name(const struct oid *o, struct oidname *on)
 	on->oid = *o;
 }
 
-/* Populates the name and labels of an OID to a buffer. */
-static void
+/*
+ * Populates the name and labels of an OID to a buffer.  If an invalid label is
+ * encountered, we'll skip the metric entirely to avoid outputting ambiguous
+ * metrics and emit the relevant details to stderr for correction.
+ *
+ * Returns true if the metric is valid, false otherwise.
+ */
+static bool
 oid_get_metric(const struct oidname *on, const struct oidformat *of,
     char *metric, size_t mlen)
 {
@@ -409,10 +415,15 @@ oid_get_metric(const struct oidname *on, const struct oidformat *of,
 	separator = '{';
 	for (i = 0; i < on->oid.len; ++i) {
 		if (*label != '\0') {
-			assert(label[strspn(label,
+			if (label[strspn(label,
 			    "abcdefghijklmnopqrstuvwxyz"
 			    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
-			    "0123456789_")] == '\0');
+			    "0123456789_")] != '\0') {
+				warnx("bad label for metric '%s': %s",
+				    metric, label);
+				return (false);
+			}
+
 			snprintf(buf, sizeof(buf), "%c%s=\"", separator, label);
 			strlcat(metric, buf, mlen);
 			while (*name != '\0') {
@@ -430,6 +441,8 @@ oid_get_metric(const struct oidname *on, const struct oidformat *of,
 	}
 	if (separator != '{')
 		strlcat(metric, "}", mlen);
+
+	return (true);
 }
 
 /* Returns whether the OID name has any labels associated to it. */
@@ -486,7 +499,14 @@ oiddescription_print(const struct oiddescription *od, FILE *fp)
 	fprintf(fp, "%s", od->description);
 }
 
-static void
+/*
+ * Print the given oid, subject to include/exclude rules.  Returns true if the
+ * oid was skipped or printed, false if an error was encountered.
+ *
+ * For the purposes of the exporter, we don't consider it an error to be unable
+ * to fetch format/value information.
+ */
+static bool
 oid_print(const struct oid *o, struct oidname *on, bool print_description,
     bool exclude, bool include, FILE *fp)
 {
@@ -497,16 +517,17 @@ oid_print(const struct oid *o, struct oidname *on, bool print_description,
 	bool has_desc;
 
 	if (!oid_get_format(o, &of) || !oid_get_value(o, &of, &ov))
-		return;
+		return (true);
 	oid_get_name(o, on);
 
-	oid_get_metric(on, &of, metric, sizeof(metric));
+	if (!oid_get_metric(on, &of, metric, sizeof(metric)))
+		return (false);
 
 	if (exclude && regexec(&exc_regex, metric, 0, NULL, 0) == 0)
-		return;
+		return (true);
 
 	if (include && regexec(&inc_regex, metric, 0, NULL, 0) != 0)
-		return;
+		return (true);
 
 	has_desc = oid_get_description(o, &od);
 	/*
@@ -514,7 +535,7 @@ oid_print(const struct oid *o, struct oidname *on, bool print_description,
 	 * redundant ZFS sysctls whose names alias with the non-legacy versions.
 	 */
 	if (has_desc && strnstr(od.description, "(LEGACY)", BUFSIZ) != NULL)
-		return;
+		return (true);
 	/*
 	 * Print the line with the description. Prometheus expects a
 	 * single unique description for every metric, which cannot be
@@ -534,6 +555,7 @@ oid_print(const struct oid *o, struct oidname *on, bool print_description,
 	fputc(' ', fp);
 	oidvalue_print(&ov, fp);
 	fputc('\n', fp);
+	return (true);
 }
 
 /* Gzip compresses a buffer of memory. */
@@ -573,7 +595,7 @@ main(int argc, char *argv[])
 	char *http_buf;
 	FILE *fp;
 	size_t http_buflen;
-	int ch, error;
+	int ch, error, failed = 0;
 	bool exclude, include, gzip_mode, http_mode, print_descriptions;
 	char errbuf[BUFSIZ];
 
@@ -631,7 +653,9 @@ main(int argc, char *argv[])
 		/* Print all OIDs. */
 		oid_get_root(&o);
 		do {
-			oid_print(&o, &on, print_descriptions, exclude, include, fp);
+			if (!oid_print(&o, &on, print_descriptions, exclude,
+			    include, fp))
+				failed++;
 		} while (oid_get_next(&o, &o));
 	} else {
 		int i;
@@ -651,7 +675,9 @@ main(int argc, char *argv[])
 			}
 			o = root;
 			do {
-				oid_print(&o, &on, print_descriptions, exclude, include, fp);
+				if (!oid_print(&o, &on, print_descriptions,
+				    exclude, include, fp))
+					failed++;
 			} while (oid_get_next(&o, &o) &&
 			    oid_is_beneath(&o, &root));
 		}
@@ -702,5 +728,5 @@ main(int argc, char *argv[])
 			}
 		}
 	}
-	return (0);
+	return (failed != 0 ? 1 : 0);
 }