Abort startup on a config file pixa cannot read (closes #176)

Of the places pixa looks for its config file on its own, it passed over
any place where os.Stat failed, so a file in a directory pixa may not
enter was skipped without a word and pixa started on a later file or on
the environment and defaults. Now only a file that does not exist is
passed over; any other error aborts startup naming the file, as a file
that does not parse already did. README.md says so where it gives the
search order.

Model: opus-5-5

README.md

@@ -354,10 +354,10 @@ pixa finds: `/etc/pixa/config.yml`, `/etc/pixa/config.yaml`,
 `~/.config/pixa/config.yml`, `~/.config/pixa/config.yaml`, then `config.yml`
 and `config.yaml` in the working directory. A named file that does not exist,
 cannot be read or does not parse aborts startup. Of the files pixa looks for on
-its own, one it finds but cannot read or parse aborts startup; one it cannot
-find, for any reason, is passed over without a message, even when the file is
-there in a directory pixa may not enter. With no file, pixa uses the environment
-and the defaults.
+its own, only one that does not exist is passed over, without a message. One
+that pixa cannot read or parse aborts startup, naming the file. So does one in a
+directory pixa may not enter, whether or not it is there, since pixa cannot
+tell. With no file, pixa uses the environment and the defaults.
 
 | Variable                             | Config key                      | Meaning                                                                      |
 | ------------------------------------ | ------------------------------- | ---------------------------------------------------------------------------- |

TODO.md

@@ -29,6 +29,11 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 a config file pixa cannot read aborts startup (closes #176): of the
+  places pixa looks for its config file on its own, only one where the file does
+  not exist is passed over; any other error, such as a directory on the path
+  that pixa may not enter, aborts startup naming the file, as a file that does
+  not parse already did.
 - 2026-10-04 request IDs returned and passed on, and `/v1/e/` revalidates
   (closes #84): pixa's own `RequestID` middleware, in place of chi's, gives each
   request an ID, its own `X-Request-ID` when that is at most 64 letters, digits,

internal/config/config.go

@@ -4,6 +4,7 @@ package config
 import (
 	"errors"
 	"fmt"
+	"io/fs"
 	"log/slog"
 	"math"
 	"net/netip"
@@ -778,10 +779,17 @@ func loadConfigFile(log *slog.Logger, appName string) (*smartconfig.Config, erro
 	for _, path := range configPaths {
 		cleanPath := filepath.Clean(path)
 
+		// Only a config file that does not exist is skipped. One that
+		// cannot be read or does not parse is a fatal startup error.
 		_, statErr := os.Stat(cleanPath)
-		if statErr == nil {
-			// A config file that exists but does not parse is a fatal
-			// startup error, never something to skip over.
+		if errors.Is(statErr, fs.ErrNotExist) {
+			continue
+		}
+
+		if statErr != nil {
+			return nil, fmt.Errorf("failed to read config file %s: %w", path, statErr)
+		}
+
 		sc, err := smartconfig.NewFromConfigPath(path)
 		if err != nil {
 			return nil, fmt.Errorf("failed to parse config file %s: %w", path, err)
@@ -791,7 +799,6 @@ func loadConfigFile(log *slog.Logger, appName string) (*smartconfig.Config, erro
 
 		return sc, nil
 	}
-	}
 
 	return nil, nil //nolint:nilnil // nil config is valid (use defaults)
 }