add FileExists and DirExists for checking what is at a path

FileExists reports whether a path holds a regular file and DirExists whether it
holds a directory, both answering false for anything they cannot look at.
Comes with doc comments and table-driven tests. (closes #15)

Model: opus-5

atomicwrite.go

@@ -1,56 +0,0 @@
-package util
-
-import (
-	"os"
-	"path/filepath"
-)
-
-// AtomicWriteFile writes data to path without ever leaving a half-written file
-// behind. It writes to a temporary file in the same directory, flushes it to
-// disk, gives it the mode in perm and renames it over path, so anything reading
-// path sees either the previous contents or the complete new contents and
-// never something in between. If any step fails, the temporary file is removed
-// and path is left as it was.
-//
-// The temporary file has to live in the same directory as path, because a
-// rename across two filesystems is not possible and would not be instant if it
-// were.
-//
-// Two details differ from os.WriteFile. The mode is applied after the file is
-// created, so perm is what the finished file ends up with rather than perm with
-// the process umask taken out of it. And the directory holding the file is not
-// flushed, so a machine that loses power immediately after this returns may
-// come back with the rename undone, even though the data itself was written.
-func AtomicWriteFile(path string, data []byte, perm os.FileMode) error {
-	directory := filepath.Dir(path)
-
-	temporary, err := os.CreateTemp(directory, "."+filepath.Base(path)+".tmp")
-	if err != nil {
-		return err
-	}
-	temporaryPath := temporary.Name()
-
-	// Does nothing once the rename below has succeeded, because by then the
-	// temporary name no longer refers to anything.
-	defer os.Remove(temporaryPath)
-
-	if _, err := temporary.Write(data); err != nil {
-		temporary.Close()
-		return err
-	}
-
-	if err := temporary.Sync(); err != nil {
-		temporary.Close()
-		return err
-	}
-
-	if err := temporary.Close(); err != nil {
-		return err
-	}
-
-	if err := os.Chmod(temporaryPath, perm); err != nil {
-		return err
-	}
-
-	return os.Rename(temporaryPath, path)
-}

atomicwrite_test.go

@@ -1,110 +0,0 @@
-package util
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-func TestAtomicWriteFile(t *testing.T) {
-	tests := []struct {
-		name     string
-		existing string
-		data     []byte
-		perm     os.FileMode
-	}{
-		{"a new file", "", []byte("hello"), 0644},
-		{"replacing a shorter file", "old", []byte("a much longer set of contents"), 0644},
-		{"replacing a longer file", "a much longer set of contents", []byte("new"), 0644},
-		{"an empty payload", "something", []byte{}, 0644},
-		{"a nil payload", "something", nil, 0644},
-		{"a private mode", "", []byte("secret"), 0600},
-		{"an executable mode", "", []byte("#!/bin/sh\n"), 0755},
-		{"binary contents", "", []byte{0x00, 0xff, 0x10, 0x00}, 0644},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			directory := t.TempDir()
-			path := filepath.Join(directory, "target")
-
-			if test.existing != "" {
-				if err := os.WriteFile(path, []byte(test.existing), 0666); err != nil {
-					t.Fatalf("could not write the file to be replaced: %v", err)
-				}
-			}
-
-			if err := AtomicWriteFile(path, test.data, test.perm); err != nil {
-				t.Fatalf("did not expect an error, got %v", err)
-			}
-
-			written, err := os.ReadFile(path)
-			if err != nil {
-				t.Fatalf("could not read the file back: %v", err)
-			}
-			if string(written) != string(test.data) {
-				t.Errorf("expected contents %q got %q", string(test.data), string(written))
-			}
-
-			info, err := os.Stat(path)
-			if err != nil {
-				t.Fatalf("could not look at the file: %v", err)
-			}
-			if info.Mode().Perm() != test.perm {
-				t.Errorf("expected mode %v got %v", test.perm, info.Mode().Perm())
-			}
-
-			entries, err := os.ReadDir(directory)
-			if err != nil {
-				t.Fatalf("could not list the directory: %v", err)
-			}
-			if len(entries) != 1 {
-				names := make([]string, 0, len(entries))
-				for _, entry := range entries {
-					names = append(names, entry.Name())
-				}
-				t.Errorf("expected only the target file to be left, got %v", names)
-			}
-		})
-	}
-}
-
-func TestAtomicWriteFileReportsAMissingDirectory(t *testing.T) {
-	path := filepath.Join(t.TempDir(), "not-there", "target")
-
-	if err := AtomicWriteFile(path, []byte("hello"), 0644); err == nil {
-		t.Errorf("expected an error for a directory that does not exist")
-	}
-}
-
-func TestAtomicWriteFileLeavesTheOldFileAloneOnFailure(t *testing.T) {
-	directory := t.TempDir()
-	path := filepath.Join(directory, "target")
-
-	if err := os.WriteFile(path, []byte("original"), 0644); err != nil {
-		t.Fatalf("could not write the file to be replaced: %v", err)
-	}
-
-	// A directory that cannot be written to means the temporary file cannot
-	// be created, so the write has to fail before anything is replaced.
-	if err := os.Chmod(directory, 0500); err != nil {
-		t.Fatalf("could not change the directory mode: %v", err)
-	}
-	defer os.Chmod(directory, 0700)
-
-	if os.Geteuid() == 0 {
-		t.Skip("running as root, which ignores the directory mode")
-	}
-
-	if err := AtomicWriteFile(path, []byte("replacement"), 0644); err == nil {
-		t.Fatalf("expected an error for a directory that cannot be written to")
-	}
-
-	existing, err := os.ReadFile(path)
-	if err != nil {
-		t.Fatalf("could not read the file back: %v", err)
-	}
-	if string(existing) != "original" {
-		t.Errorf("expected the original contents to survive, got %q", string(existing))
-	}
-}

exists.go

@@ -0,0 +1,31 @@
+package util
+
+import "os"
+
+// FileExists reports whether there is a regular file at path. A directory, a
+// device node or a socket gives false, and so does anything the process is not
+// allowed to look at, since from the caller's point of view there is no usable
+// file there either way.
+//
+// Symbolic links are followed, so a link pointing at a regular file gives true
+// and a link pointing at nothing gives false.
+func FileExists(path string) bool {
+	info, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+
+	return info.Mode().IsRegular()
+}
+
+// DirExists reports whether there is a directory at path. As with FileExists,
+// anything that cannot be looked at gives false, and symbolic links are
+// followed.
+func DirExists(path string) bool {
+	info, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+
+	return info.IsDir()
+}

exists_test.go

@@ -0,0 +1,57 @@
+package util
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestFileExistsAndDirExists(t *testing.T) {
+	root := t.TempDir()
+
+	file := filepath.Join(root, "a-file")
+	if err := os.WriteFile(file, []byte("contents"), 0644); err != nil {
+		t.Fatalf("could not write the test file: %v", err)
+	}
+
+	directory := filepath.Join(root, "a-directory")
+	if err := os.Mkdir(directory, 0755); err != nil {
+		t.Fatalf("could not make the test directory: %v", err)
+	}
+
+	linkToFile := filepath.Join(root, "link-to-file")
+	if err := os.Symlink(file, linkToFile); err != nil {
+		t.Fatalf("could not make the test link: %v", err)
+	}
+
+	brokenLink := filepath.Join(root, "broken-link")
+	if err := os.Symlink(filepath.Join(root, "nothing-here"), brokenLink); err != nil {
+		t.Fatalf("could not make the broken test link: %v", err)
+	}
+
+	tests := []struct {
+		name          string
+		path          string
+		expectedFile  bool
+		expectedIsDir bool
+	}{
+		{"a regular file", file, true, false},
+		{"a directory", directory, false, true},
+		{"a link to a file", linkToFile, true, false},
+		{"a link to nothing", brokenLink, false, false},
+		{"a path that is not there", filepath.Join(root, "missing"), false, false},
+		{"a path whose parent is not there", filepath.Join(root, "missing", "deeper"), false, false},
+		{"the empty path", "", false, false},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := FileExists(test.path); got != test.expectedFile {
+				t.Errorf("FileExists: expected %v got %v", test.expectedFile, got)
+			}
+			if got := DirExists(test.path); got != test.expectedIsDir {
+				t.Errorf("DirExists: expected %v got %v", test.expectedIsDir, got)
+			}
+		})
+	}
+}