Refuse an exclude pattern that does not compile at config load (closes #455)

An exclude pattern that did not compile was skipped with a warning during the backup, which then stored what the user meant to leave out. Config.Validate now refuses it, naming the setting and quoting the pattern, for example exclude[2]: "build[" is not a valid exclude pattern: ...

The compile step moves to a new package, internal/exclude, as exclude.Compile; Validate and the scanner both call it. The scanner's skip-and-warn path is gone; a pattern that does not compile there now fails the scanner.

Model: opus-5-5

internal/exclude/exclude.go

@@ -0,0 +1,77 @@
+// Package exclude compiles exclude patterns into globs.
+package exclude
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+
+	"github.com/gobwas/glob"
+)
+
+var errBadExcludePattern = errors.New("is not a valid exclude pattern")
+
+// Compile compiles one exclude pattern into the globs that match it.
+// anchored reports whether the pattern matches only from the root of the
+// source directory. An empty pattern yields no globs. Config.Validate and
+// the scanner both compile patterns with it, so a pattern that loads is one
+// the scanner can use.
+func Compile(pattern string) ([]glob.Glob, bool, error) {
+	if pattern == "" {
+		return nil, false, nil
+	}
+
+	// A leading "/" anchors the pattern to the source directory.
+	trimmed, anchored := strings.CutPrefix(pattern, "/")
+
+	// A trailing "/" marks a directory and is not matched.
+	trimmed = strings.TrimSuffix(trimmed, "/")
+
+	// A leading "**/" demands a "/" before the name and so misses an
+	// entry directly under the source path. It means "at every depth",
+	// which is what an unanchored pattern already does, so drop the
+	// prefix and match unanchored: as in gitignore, "**/x" and
+	// "/**/x" then also match a top-level "x".
+	if rest, found := strings.CutPrefix(trimmed, "**/"); found {
+		trimmed = rest
+		anchored = false
+	}
+
+	// Likewise "/**/" inside a pattern demands two slashes, so
+	// "a/**/*.log" misses "a/x.log". As in gitignore it stands for
+	// zero or more directories, so also compile the pattern with it
+	// made a single "/". The glob library's {a,b} is no help: with a
+	// "**" inside it in mid-pattern, it misses paths it should match.
+	var globs []glob.Glob
+
+	for _, variant := range doubleStarVariants(trimmed) {
+		g, err := glob.Compile(variant, '/')
+		if err != nil {
+			return nil, false, fmt.Errorf("%q %w: %w",
+				pattern, errBadExcludePattern, err)
+		}
+
+		globs = append(globs, g)
+	}
+
+	return globs, anchored, nil
+}
+
+// doubleStarVariants returns pattern once for every way of keeping each
+// "/**/" in it or making it a single "/".
+func doubleStarVariants(pattern string) []string {
+	const doubleStar = "/**/"
+
+	before, after, found := strings.Cut(pattern, doubleStar)
+	if !found {
+		return []string{pattern}
+	}
+
+	var variants []string
+
+	for _, rest := range doubleStarVariants(after) {
+		variants = append(variants, before+doubleStar+rest, before+"/"+rest)
+	}
+
+	return variants
+}

internal/exclude/exclude_test.go

@@ -0,0 +1,73 @@
+package exclude_test
+
+import (
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/gobwas/glob"
+	"sneak.berlin/go/vaultik/internal/exclude"
+)
+
+// TestCompile checks the error, the anchoring and the globs that Compile
+// returns for one exclude pattern.
+func TestCompile(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name         string
+		pattern      string
+		wantErr      string
+		wantAnchored bool
+		// wantMatches lists paths that one of the globs must match. When
+		// it is empty, Compile must return no globs.
+		wantMatches []string
+	}{
+		{name: "does not compile", pattern: "build[",
+			wantErr: `"build[" is not a valid exclude pattern`},
+		{name: "empty", pattern: ""},
+		{name: "leading slash anchors", pattern: "/build",
+			wantAnchored: true, wantMatches: []string{"build"}},
+		{name: "double star after the slash clears anchoring",
+			pattern: "/**/build", wantMatches: []string{"build"}},
+		{name: "double star in the middle", pattern: "a/**/*.log",
+			wantMatches: []string{"a/x.log", "a/b/x.log"}},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			globs, anchored, err := exclude.Compile(tt.pattern)
+			if tt.wantErr == "" && err != nil {
+				t.Fatalf("Compile(%q) error = %v, want none", tt.pattern, err)
+			}
+
+			if tt.wantErr != "" &&
+				(err == nil || !strings.HasPrefix(err.Error(), tt.wantErr)) {
+				t.Fatalf("Compile(%q) error = %v, want one starting with %q",
+					tt.pattern, err, tt.wantErr)
+			}
+
+			if anchored != tt.wantAnchored {
+				t.Errorf("Compile(%q) anchored = %t, want %t",
+					tt.pattern, anchored, tt.wantAnchored)
+			}
+
+			if len(tt.wantMatches) == 0 && len(globs) != 0 {
+				t.Errorf("Compile(%q) returned %d globs, want none",
+					tt.pattern, len(globs))
+			}
+
+			for _, path := range tt.wantMatches {
+				matched := slices.ContainsFunc(globs, func(g glob.Glob) bool {
+					return g.Match(path)
+				})
+				if !matched {
+					t.Errorf("Compile(%q) globs do not match %q",
+						tt.pattern, path)
+				}
+			}
+		})
+	}
+}