fix(commands): distinguish empty query results

Track exit codes that indicate an empty package query separately from
accepted success codes.

Treat pacman exit status 1 as an empty result only when both stdout and
stderr are empty, while preserving genuine command failures as errors.

Apply the empty-result handling to pacman queries that may legitimately
return no packages, including update checks.

1 files changed, 34 insertions, 11 deletions

diff --git a/prometheus_pacman_exporter/__init__.py b/prometheus_pacman_exporter/__init__.py
index 0125187..80198d7 100644
--- a/prometheus_pacman_exporter/__init__.py
+++ b/prometheus_pacman_exporter/__init__.py
@@ -30,38 +30,48 @@ class PackageQuery(NamedTuple):
     description: str
     metric: Gauge
     accepted_exit_codes: tuple[int, ...] = (0,)
+    empty_exit_codes: tuple[int, ...] = ()
 
 
 PACMAN_BASE_COMMAND = ("pacman", "--query")
+PACMAN_EMPTY_EXIT_CODES = (1,)
 BASE_QUERIES: tuple[PackageQuery, ...] = (
     PackageQuery(PACMAN_BASE_COMMAND, "installed packages", metrics.INSTALLED_PACKAGES),
     PackageQuery(
         (*PACMAN_BASE_COMMAND, "--explicit"),
         "explicitly installed packages",
         metrics.EXPLICIT_PACKAGES,
+        empty_exit_codes=PACMAN_EMPTY_EXIT_CODES,
     ),
     PackageQuery(
         (*PACMAN_BASE_COMMAND, "--deps"),
         "packages installed as dependencies",
         metrics.DEPENDS_PACKAGES,
+        empty_exit_codes=PACMAN_EMPTY_EXIT_CODES,
     ),
     PackageQuery(
         (*PACMAN_BASE_COMMAND, "--unrequired"),
         "unrequired packages",
         metrics.UNREQUIRED_PACKAGES,
+        empty_exit_codes=PACMAN_EMPTY_EXIT_CODES,
     ),
     PackageQuery(
         (*PACMAN_BASE_COMMAND, "--foreign"),
         "foreign packages",
         metrics.FOREIGN_PACKAGES,
+        empty_exit_codes=PACMAN_EMPTY_EXIT_CODES,
     ),
     PackageQuery(
-        (*PACMAN_BASE_COMMAND, "--native"), "native packages", metrics.NATIVE_PACKAGES
+        (*PACMAN_BASE_COMMAND, "--native"),
+        "native packages",
+        metrics.NATIVE_PACKAGES,
+        empty_exit_codes=PACMAN_EMPTY_EXIT_CODES,
     ),
     PackageQuery(
         (*PACMAN_BASE_COMMAND, "--deps", "--unrequired"),
         "orphan packages",
         metrics.ORPHAN_PACKAGES,
+        (0, 1),
     ),
 )
 
@@ -124,6 +134,7 @@ def run_command(
     ctx: click.Context,
     command: tuple[str, ...],
     accepted_exit_codes: tuple[int, ...] = (0,),
+    empty_exit_codes: tuple[int, ...] = (),
 ) -> subprocess.CompletedProcess[str]:
     """Run a command and return the result.
 
@@ -133,19 +144,29 @@ def run_command(
     :param ctx: Click context injected by :func:`click.pass_context`.
     :param command: Command and its arguments.
     :param accepted_exit_codes: Exit codes considered successful.
+    :param empty_exit_codes: Exit codes considered an empty result when both
+        stdout and stderr are empty.
     :return: Completed process with stdout and stderr captured.
     """
     command_string = shlex.join(command)
-    debug("Running command", command_string, err=True)
+    debug("Running command:", command_string, err=True)
     result = subprocess.run(command, capture_output=True, encoding="utf-8", check=False)
 
-    if result.returncode not in accepted_exit_codes:
-        error(
-            f"Command ({command_string}) failed with exit code:", str(result.returncode)
-        )
-        if result.stderr:
-            error("stderr:", result.stderr.strip())
-        ctx.exit(1)
+    if result.returncode in accepted_exit_codes:
+        return result
+
+    if (
+        result.returncode in empty_exit_codes
+        and not result.stdout.strip()
+        and not result.stderr.strip()
+    ):
+        debug("Command returned an empty result:", command_string, err=True)
+        return result
+
+    error(f"Command ({command_string}) failed with exit code:", str(result.returncode))
+    if result.stderr:
+        error("stderr:", result.stderr.strip())
+    ctx.exit(1)
 
     return result
 
@@ -262,7 +283,7 @@ def prometheus_pacman_exporter(
                 (*PACMAN_BASE_COMMAND, "--upgrades"),
                 "updateable packages",
                 metrics.UPDATEABLE_PACKAGES,
-                (0, 1),
+                empty_exit_codes=PACMAN_EMPTY_EXIT_CODES,
             )
         )
 
@@ -276,7 +297,9 @@ def prometheus_pacman_exporter(
         )
 
     for query in queries:
-        output = run_command(query.command, query.accepted_exit_codes)
+        output = run_command(
+            query.command, query.accepted_exit_codes, query.empty_exit_codes
+        )
         value = len(output.stdout.splitlines())
         verbose(f"Setting {query.description}:", str(value), err=True)
         query.metric.set(value)