Select configuration files through host rules

internal/engine/discovery.go

@@ -2,6 +2,7 @@ package engine
 
 import (
 	"errors"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -459,3 +460,60 @@ func filenameGroupAt(group FilenameGroup, index int, application string) string
 		return ""
 	}
 }
+
+type discoveryHostProbe struct {
+	evalSymlinks func(string) (string, error)
+	stat         func(string) (fs.FileInfo, error)
+}
+
+func defaultDiscoveryHostProbe() discoveryHostProbe {
+	return discoveryHostProbe{evalSymlinks: filepath.EvalSymlinks, stat: os.Stat}
+}
+
+// resolveSearchSource selects one ordinary file source. A nil description leaves
+// an existing source untouched; the selected file is read later by readSource.
+func resolveSearchSource(source Source, description *Search, application string, required bool, helpers discoveryDirectoryHelpers, probe discoveryHostProbe) (Source, []Diagnostic) {
+	if description == nil {
+		return source, nil
+	}
+	iterator, diagnostics := newSearchCandidateIterator(*description, application, helpers)
+	if len(diagnostics) != 0 {
+		return Source{}, diagnostics
+	}
+	defaults := defaultDiscoveryHostProbe()
+	if probe.evalSymlinks == nil {
+		probe.evalSymlinks = defaults.evalSymlinks
+	}
+	if probe.stat == nil {
+		probe.stat = defaults.stat
+	}
+	for {
+		candidate, ok, err := iterator.next()
+		if err != nil {
+			if errors.Is(err, errUnixSystemPresetUnavailable) {
+				return Source{}, []Diagnostic{optionDiagnostic(CodeOptionsInvalid, "Unix system directory search is unavailable on this platform")}
+			}
+			return Source{}, []Diagnostic{classifySourceError(err, "")}
+		}
+		if !ok {
+			if required {
+				return Source{}, []Diagnostic{sourceDiagnostic(CodeSourceMissing, "", "required configuration search found no file")}
+			}
+			return NoSource(), nil
+		}
+		canonical, err := probe.evalSymlinks(candidate)
+		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				continue
+			}
+			return Source{}, []Diagnostic{classifySourceError(err, "")}
+		}
+		if _, err := probe.stat(canonical); err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				continue
+			}
+			return Source{}, []Diagnostic{classifySourceError(err, "")}
+		}
+		return File(candidate), nil
+	}
+}