README: guard the daily cron scan against an unmounted disk (closes #175)

A disk that is not mounted leaves its mount point an empty directory.
scan then deletes every record under it, and the next scan reads every
file again. The README now says so. Its cron line runs scan only when
mountpoint finds the disks mounted, and one sentence says which mount
points to test.

Model: opus-5-5

README.md

@@ -114,22 +114,27 @@ docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
 Run `scan` as root, so that it can read every file. `scan` warns about a
 directory it cannot read, and about a new or changed file it cannot read. It
 skips the path and deletes the records at or below it (see "Rules for the
-walk"). As the file `/etc/cron.d/sfdupes`:
+walk").
+
+An unmounted disk leaves its mount point empty, so `scan` deletes every record
+under it, and the next scan reads every file on the disk again. Test the mount
+point of each disk at or below `/srv` with its own `mountpoint -q`, joined with
+`&&`, and test `/srv` only when it is one. As the file `/etc/cron.d/sfdupes`:
 
 ```
-30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
+30 3 * * * root if mountpoint -q /srv; then /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log; else echo "a disk is not mounted; scan of /srv skipped"; fi
 ```
 
 - The first scan creates `/var/lib/sfdupes/db.sqlite` and its directory. To keep
-  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` before the
-  command on the same line.
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
+  before `/usr/local/bin/sfdupes`.
 - `scan` writes nothing to stdout. Its stderr, appended here to
   `/var/log/sfdupes.log`, holds:
     - one line naming the database;
     - progress lines, as described under "Progress";
     - a warning for each path it skips;
-    - a line for each system, temporary or cache directory the walk leaves out
-      (see "Rules for the walk");
+    - a line for each system or temporary directory the walk leaves out (see
+      "Rules for the walk");
     - a line for each path left out because its filesystem is one the kernel,
       the core of the operating system, makes up or keeps in memory, such as
       `tmpfs`;
@@ -300,9 +305,8 @@ The reference below is also the program's specification.
   of directories on other filesystems, as the same flag does for `du` and
   `rsync`. By default the walk also enters directories on other filesystems.
 - `--no-default-excludes` (`scan`, `dwim`) lets the walk enter the default
-  excludes. Those are the system, temporary and cache directories the walk
-  otherwise leaves out, such as `/var/tmp` and `/var/cache` on Linux (see "Rules
-  for the walk").
+  excludes. Those are the system and temporary directories the walk otherwise
+  leaves out, such as `/var/tmp` on Linux (see "Rules for the walk").
 - `--exclude PATH` (`scan`, `dwim`) keeps the walk out of `PATH` and everything
   below it. `PATH` may be a directory or a file. The flag may be given more than
   once. The walk knows `PATH` by its device ID and inode number, as it knows the
@@ -572,17 +576,12 @@ walk practical on large or busy storage pools.
     `--no-default-excludes` does not change this rule.
 
 - Below an operand, the walk does not enter a default exclude. On Linux, the
-  default excludes hold the kernel's view of the running system, temporary
-  files, and caches. A cache here is data that programs and package managers
-  download or build again on their own. The default excludes depend on the
-  system, and `sfdupes scan --help` lists those of the system it runs on. In the
-  lists, `*` stands for any one directory name, so `/home/*/.cache` matches
-  `/home/alice/.cache`. On Linux they are:
+  default excludes hold temporary files and the kernel's view of the running
+  system. They depend on the system, and `sfdupes scan --help` lists those of
+  the system it runs on. On Linux they are:
 
     ```
-    /proc, /sys, /dev, /run, /tmp, /var/tmp, /var/cache
-    /root/.cache, /root/.npm, /root/.cargo/registry, /root/go/pkg/mod
-    /home/*/.cache, /home/*/.npm, /home/*/.cargo/registry, /home/*/go/pkg/mod
+    /proc, /sys, /dev, /run, /tmp, /var/tmp
     ```
 
     On macOS they are:
@@ -590,21 +589,13 @@ walk practical on large or busy storage pools.
     ```
     /dev, /private/tmp, /private/var/tmp, /private/var/vm
     /System/Volumes/VM, /System/Volumes/Data
-    /private/var/folders, /Library/Caches
-    /private/var/root/Library/Caches, /private/var/root/.cache
-    /private/var/root/.npm, /private/var/root/.cargo/registry
-    /private/var/root/go/pkg/mod
-    /Users/*/Library/Caches, /Users/*/.cache, /Users/*/.npm
-    /Users/*/.cargo/registry, /Users/*/go/pkg/mod
     ```
 
-    `/private/var/folders` holds each user's temporary files and caches on
-    macOS. Root's home on macOS is `/var/root`, which the walk reaches as
-    `/private/var/root`. A walk of `/` on macOS reaches everything in
-    `/System/Volumes/Data` already, through firmlinks. Firmlinks are the links
-    macOS uses to show the data volume inside the system volume. Walking
-    `/System/Volumes/Data` as well would record each of those files twice. Any
-    other system has no default excludes.
+    A walk of `/` on macOS reaches everything in `/System/Volumes/Data` already,
+    through firmlinks. Firmlinks are the links macOS uses to show the data
+    volume inside the system volume. Walking `/System/Volumes/Data` as well
+    would record each of those files twice. Any other system has no default
+    excludes.
 
     An operand at or inside a default exclude is walked, so
     `sfdupes scan /tmp/x` works when `/tmp/x` is on a persistent filesystem.

internal/sfdupes/exclude.go

@@ -72,31 +72,14 @@ type walkRules struct {
 func defaultExcludes(goos string) []string {
 	switch goos {
 	case goosLinux:
-		// The kernel's view of the running system, temporary files, and
-		// caches that programs and package managers fill again on their
-		// own: the system's, root's and those in each home in /home.
-		return []string{
-			"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp", "/var/cache",
-			"/root/.cache", "/root/.npm", "/root/.cargo/registry",
-			"/root/go/pkg/mod",
-			"/home/*/.cache", "/home/*/.npm", "/home/*/.cargo/registry",
-			"/home/*/go/pkg/mod",
-		}
+		return []string{"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"}
 	case goosDarwin:
 		// A walk of / reaches everything in /System/Volumes/Data through
 		// the firmlinks, so walking it as well would record each of
-		// those files twice. /private/var/folders holds each user's
-		// temporary files and caches. The walk reaches root's home,
-		// /var/root, as /private/var/root.
+		// those files twice.
 		return []string{
 			"/dev", "/private/tmp", "/private/var/tmp", "/private/var/vm",
 			"/System/Volumes/VM", "/System/Volumes/Data",
-			"/private/var/folders", "/Library/Caches",
-			"/private/var/root/Library/Caches", "/private/var/root/.cache",
-			"/private/var/root/.npm", "/private/var/root/.cargo/registry",
-			"/private/var/root/go/pkg/mod",
-			"/Users/*/Library/Caches", "/Users/*/.cache", "/Users/*/.npm",
-			"/Users/*/.cargo/registry", "/Users/*/go/pkg/mod",
 		}
 	case goosWindows:
 		// Relative to the root of the drive an operand lies on. The

internal/sfdupes/exclude_test.go

@@ -24,22 +24,10 @@ func TestDefaultExcludes(t *testing.T) {
 	// on any other system. The Windows list exists in the code only: the
 	// binary does not build for Windows, so the README leaves it out.
 	cases := map[string][]string{
-		goosLinux: {
-			"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp", "/var/cache",
-			"/root/.cache", "/root/.npm", "/root/.cargo/registry",
-			"/root/go/pkg/mod",
-			"/home/*/.cache", "/home/*/.npm", "/home/*/.cargo/registry",
-			"/home/*/go/pkg/mod",
-		},
+		goosLinux: {"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"},
 		goosDarwin: {
 			"/dev", "/private/tmp", "/private/var/tmp", "/private/var/vm",
 			"/System/Volumes/VM", "/System/Volumes/Data",
-			"/private/var/folders", "/Library/Caches",
-			"/private/var/root/Library/Caches", "/private/var/root/.cache",
-			"/private/var/root/.npm", "/private/var/root/.cargo/registry",
-			"/private/var/root/go/pkg/mod",
-			"/Users/*/Library/Caches", "/Users/*/.cache", "/Users/*/.npm",
-			"/Users/*/.cargo/registry", "/Users/*/go/pkg/mod",
 		},
 		goosWindows: {
 			"pagefile.sys", "swapfile.sys", "hiberfil.sys",
@@ -75,29 +63,6 @@ func TestIsDefaultExclude(t *testing.T) {
 			t.Errorf("isDefaultExclude(%q) = %v, want %v", path, got, want)
 		}
 	}
-
-	// The Linux list matches the cache directories in /root and in each
-	// home in /home, but nothing below them and no .cache directory
-	// elsewhere.
-	linux := map[string]bool{
-		"/home/u/.cache":           true,
-		"/home/u/go/pkg/mod":       true,
-		"/root/.cache":             true,
-		"/var/cache":               true,
-		"/home/u/.cache/pip":       false,
-		"/home/u/src/.cache":       false,
-		"/srv/home/u/.cache":       false,
-		"/home/u/.cargo/bin":       false,
-		"/home/u/go/pkg/mod/cache": false,
-	}
-
-	for path, want := range linux {
-		got := isDefaultExclude(path, defaultExcludes(goosLinux))
-		if got != want {
-			t.Errorf("isDefaultExclude(%q, Linux list) = %v, want %v", path,
-				got, want)
-		}
-	}
 }
 
 func TestReadMountInfo(t *testing.T) {
@@ -325,7 +290,7 @@ func TestRunScanTempDirOperand(t *testing.T) {
 	t.Setenv(databaseEnv, path)
 
 	dir := realTempDir(t)
-	if !insideDefaultExclude(dir, defaultExcludes(runtime.GOOS)) {
+	if !underAnyRoot(dir, defaultExcludes(runtime.GOOS)) {
 		t.Skip(dir + " is not inside a default exclude")
 	}
 
@@ -746,20 +711,6 @@ func shmTempDir(t *testing.T) (string, string, walkRules) {
 	return dir, fsType, walkRules{nonPersistent: nonPersistent}
 }
 
-// insideDefaultExclude reports whether path, which is absolute and
-// clean, or a directory above it is one of excludes.
-func insideDefaultExclude(path string, excludes []string) bool {
-	for dir := path; ; dir = filepath.Dir(dir) {
-		if isDefaultExclude(dir, excludes) {
-			return true
-		}
-
-		if dir == "/" {
-			return false
-		}
-	}
-}
-
 // assertRecorded checks that the database at dbPath holds a record for
 // path when want is set, and none when it is not.
 func assertRecorded(t *testing.T, dbPath, path string, want bool) {