README.md

@@ -94,22 +94,24 @@ Adds the `pub` line to `~/.ssh/authorized_keys` on the host. No command is run
 on the host: the file is fetched, changed here, and written back with the
 system `sftp` client in batch mode.
 
-The first connection fetches `~/.ssh/authorized_keys`. The file reads as empty
-only when `sftp` reported that file as not being there — the one line naming
-that path. The same wording anywhere else in the session does not count: `ssh`
-writes `No such file or directory` about an `-i` it cannot find, on a session
-that then authenticates through the agent. When `sftp` failed for any other
-reason — the file is there and cannot be read, the connection did not come up —
-the tool prints what `sftp` said and exits with status 1 without writing
-anything, rather than put a file back holding the new key alone. What `sftp`
-cannot tell apart is a missing file and one in a directory it cannot enter, so a
-`~/.ssh` whose mode shuts the user out reads as a host with no file; the second
-connection sets that mode to `0700` and writes, as on a host that has none. If
-an identical line is already in the file, the tool prints
-`already present` and connects no further. Otherwise the line is added (after a
-newline, if the file did not end with one) and a second connection:
+The first connection lists `~/.ssh` and then fetches
+`~/.ssh/authorized_keys` from it. The file reads as empty in two cases only:
+`sftp` reported `~/.ssh` itself as not being there, or the listing came up and
+the file was not in it. Any other outcome of that connection fails the run — a
+`~/.ssh` that is there but cannot be entered, an `authorized_keys` that is there
+but cannot be read, or a connection that did not come up — and the tool prints
+what `sftp` said and exits with status 1 without writing anything, rather than
+put a file back holding the new key alone. The listing is what tells a missing
+directory from one shut to the user, which `sftp` reports on a fetch the same
+way; the wording of a missing file elsewhere does not count either, since `ssh`
+writes `No such file or directory` about an `-i` it cannot find on a session
+that then authenticates through the agent. If an identical line is already in
+the file, the tool prints `already present` and connects no further. Otherwise
+the line is added (after a newline, if the file did not end with one) and a
+second connection:
 
-- creates `~/.ssh` and sets it to mode `0700`;
+- makes `~/.ssh` and sets it to mode `0700`, but only when the first connection
+  found none; a `~/.ssh` that was already there keeps the mode it had;
 - uploads the new file as `~/.ssh/authorized_keys.keyfunc-<random>` and sets it
   to mode `0600`;
 - renames that file over `~/.ssh/authorized_keys`.

internal/cli/ssh/install.go

@@ -76,7 +76,7 @@ func add(cmd *cobra.Command, host string, options []string, line string) error {
 
 	defer func() { _ = os.RemoveAll(work) }()
 
-	content, err := fetch(cmd, host, options,
+	content, present, err := fetch(cmd, host, options,
 		filepath.Join(work, "authorized_keys"),
 	)
 	if err != nil {
@@ -88,16 +88,18 @@ func add(cmd *cobra.Command, host string, options []string, line string) error {
 		return write(cmd, "already present\n")
 	}
 
-	return upload(cmd, host, options, work, merged)
+	return upload(cmd, host, options, work, merged, present)
 }
 
 // upload writes the new file to the host and renames it over
 // authorized_keys, which is the step that either happens or does not.
 // Nothing is removed when a step fails: the file left behind is named
-// so that it can be looked at and cleared away by hand.
+// so that it can be looked at and cleared away by hand. The directory
+// is made and set to its mode only when the read found none: an .ssh
+// that was already there is left with the mode it had.
 func upload(
 	cmd *cobra.Command, host string, options []string,
-	work, merged string,
+	work, merged string, present bool,
 ) error {
 	local := filepath.Join(work, "authorized_keys.merged")
 
@@ -111,14 +113,24 @@ func upload(
 		return err
 	}
 
-	// The mkdir may fail: the directory is usually there already.
-	said, err := session(cmd, host, options, []string{
-		"-mkdir " + directory,
-		"chmod " + directoryMode + " " + directory,
-		"put " + quoted(local) + " " + sidecar,
-		"chmod " + fileMode + " " + sidecar,
-		"rename " + sidecar + " " + authorized,
-	})
+	var batch []string
+
+	if !present {
+		// The mkdir is allowed to fail in case the directory appeared
+		// between the read and now; the chmod then sets its mode.
+		batch = append(batch,
+			"-mkdir "+directory,
+			"chmod "+directoryMode+" "+directory,
+		)
+	}
+
+	batch = append(batch,
+		"put "+quoted(local)+" "+sidecar,
+		"chmod "+fileMode+" "+sidecar,
+		"rename "+sidecar+" "+authorized,
+	)
+
+	said, err := session(cmd, host, options, batch)
 	if err != nil {
 		// sftp echoes each command as it runs it and stops at the
 		// first that fails, so the name is in what it said only once
@@ -185,31 +197,82 @@ func merge(content, line string) (string, bool) {
 }
 
 // fetch brings the host's authorized_keys into the given path and
-// returns what is in it. A host that has no such file reads as empty,
-// but only when that is what sftp said about it: a file that is there
-// and cannot be read fails the run, because writing back over it
-// would leave the host with the new key and nothing else.
+// returns what is in it, and whether the .ssh directory was already
+// there. The one session lists .ssh and then gets the file, so the
+// listing settles the state of the directory before the get is read.
+//
+// The file reads as empty in just two cases: sftp reported .ssh itself
+// as not there, or the listing succeeded and the get then reported the
+// file as not there. Anything else — the listing refused, the file
+// there but unreadable, the connection down — fails the run and writes
+// nothing, because writing back over what was not read would leave the
+// host with the new key and nothing else. sftp cannot tell a missing
+// file from one in a directory it cannot enter, so the listing does:
+// a directory that is there but cannot be read is a failure, not an
+// empty file.
 func fetch(
 	cmd *cobra.Command, host string, options []string, into string,
-) (string, error) {
+) (string, bool, error) {
 	said, err := session(cmd, host, options, []string{
+		"ls -1 " + directory,
 		"get " + authorized + " " + quoted(into),
 	})
 	if err != nil {
-		if absent(said) {
-			return "", nil
+		if directoryAbsent(said) {
+			return "", false, nil
 		}
 
-		return "", err
+		if absent(said) {
+			return "", true, nil
+		}
+
+		return "", false, err
 	}
 
 	//nolint:gosec // the path is a temporary file of the tool's own
 	content, err := os.ReadFile(into)
 	if err != nil {
-		return "", fmt.Errorf("reading the fetched file: %w", err)
+		return "", false, fmt.Errorf("reading the fetched file: %w", err)
 	}
 
-	return string(content), nil
+	return string(content), true, nil
+}
+
+// directoryAbsent says whether sftp reported .ssh itself as not being
+// there, which is the one listing failure read as a host that has no
+// authorized_keys yet. The reading is taken only from the line in which
+// sftp reports on that directory: any other failure of the listing, in
+// particular a directory that is there but cannot be entered, is left
+// as a failure, so that no key is written to a host whose keys were
+// never read.
+func directoryAbsent(said string) bool {
+	for line := range strings.Lines(said) {
+		named, is := reportedCannotList(strings.TrimSpace(line))
+		if is && (named == directory ||
+			strings.HasSuffix(named, "/"+directory)) {
+			return true
+		}
+	}
+
+	return false
+}
+
+// reportedCannotList returns the path an sftp line reports it cannot
+// list for want of the directory, and whether the line is such a
+// report. The client writes this one wording when the directory a
+// listing names is not there, giving the path the server expanded.
+func reportedCannotList(line string) (string, bool) {
+	const (
+		before = `Can't ls: "`
+		after  = `" not found`
+	)
+
+	if !strings.HasPrefix(line, before) ||
+		!strings.HasSuffix(line, after) {
+		return "", false
+	}
+
+	return strings.TrimSuffix(strings.TrimPrefix(line, before), after), true
 }
 
 // absent says whether sftp reported the file that was asked for as

internal/cli/ssh/install_test.go

@@ -10,6 +10,7 @@ import "testing"
 const (
 	echoed = `sftp> get .ssh/authorized_keys "/tmp/keyfunc/authorized_keys"
 `
+	listed  = "sftp> ls -1 .ssh\n"
 	warning = `Warning: Identity file /gone not accessible: ` +
 		"No such file or directory.\n"
 )
@@ -76,3 +77,57 @@ func TestAbsenceIsReadOnlyFromWhatSFTPSaidAboutAuthorizedKeys(t *testing.T) {
 		})
 	}
 }
+
+// TestTheDirectoryIsReadAsAbsentOnlyFromTheListingSayingSo holds the
+// wordings the OpenSSH client was seen to use when a listing fails: a
+// directory it cannot find is reported one way, and one it cannot enter
+// another, and only the first is read as a host with no .ssh yet.
+func TestTheDirectoryIsReadAsAbsentOnlyFromTheListingSayingSo(t *testing.T) {
+	t.Parallel()
+
+	listings := map[string]struct {
+		said string
+		want bool
+	}{
+		"the directory is not there": {
+			said: listed + `Can't ls: "/home/someone/.ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is not there, named as it was asked for": {
+			said: listed + `Can't ls: ".ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is not there and an identity file is not either": {
+			said: warning + listed +
+				`Can't ls: "/home/someone/.ssh" not found` + "\n",
+			want: true,
+		},
+		"the directory is there and cannot be entered": {
+			said: listed +
+				`remote readdir("/home/someone/.ssh/"): Permission denied` + "\n",
+			want: false,
+		},
+		"some other directory is not there": {
+			said: listed + `Can't ls: "/home/someone/.config" not found` + "\n",
+			want: false,
+		},
+		"the connection did not come up": {
+			said: "ssh: connect to host example.com port 22: " +
+				"Connection refused\nConnection closed\n",
+			want: false,
+		},
+	}
+
+	for name, listing := range listings {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			if directoryAbsent(listing.said) != listing.want {
+				t.Errorf(
+					"read as absent: %t, wanted %t, from:\n%s",
+					!listing.want, listing.want, listing.said,
+				)
+			}
+		})
+	}
+}

internal/cli/ssh_test.go

@@ -68,13 +68,18 @@ const keyLine = vectorZero + " keyfunc/ssh/0\n"
 // work. A command that begins with a dash may fail; any other failure
 // ends the session, as it does in sftp's own batch mode.
 //
-// The two ways a get can fail are worded as the OpenSSH client words
-// them, both naming the path the server expanded: a file that is not
-// there, which is the one failure the tool reads as an empty file, and
-// a file that is there and cannot be read, which is not. An -i naming
-// a file that is not here draws the warning ssh writes for it, which
-// carries the wording of a missing file into a session that goes on to
-// authenticate.
+// The listing and the two ways a get can fail are worded as the
+// OpenSSH client words them, each naming the path the server expanded.
+// A listing fails one way when .ssh is not there and another when it is
+// there but shut to the user; the first is the only failure read as a
+// host with no file. A get fails one way for a file that is not there,
+// which after a listing that came up empty is also read as no file, and
+// another for a file that is there and cannot be read, which is a
+// failure. A directory shut to the user is stood in for by mode 000,
+// which the listing reads off the mode itself so that the test does not
+// turn on the user it runs as. An -i naming a file that is not here
+// draws the warning ssh writes for it, which carries the wording of a
+// missing file into a session that goes on to authenticate.
 const installer = `
 previous=
 for argument in "$@"; do
@@ -99,6 +104,23 @@ while IFS= read -r line; do
 	eval "set -- $line"
 	worked=yes
 	case "$1" in
+	ls)
+		dir=$2
+		[ "$dir" = -1 ] && dir=$3
+		if [ ! -e "$home/$dir" ]; then
+			worked=no
+			printf 'Can'\''t ls: "%s" not found\n' "$home/$dir" >&2
+		elif [ -d "$home/$dir" ] && [ "$(stat -c '%a' "$home/$dir")" = 0 ]; then
+			worked=no
+			printf 'remote readdir("%s/"): Permission denied\n' \
+				"$home/$dir" >&2
+		else
+			for entry in "$home/$dir"/*; do
+				[ -e "$entry" ] || continue
+				printf '%s/%s\n' "$dir" "$(basename "$entry")"
+			done
+		fi
+		;;
 	get)
 		if [ ! -e "$home/$2" ]; then
 			worked=no
@@ -176,8 +198,8 @@ func TestAKeyThatIsAlreadyThereIsLeftAlone(t *testing.T) {
 	require.Equal(t, "already present\n", install(t, host))
 	require.Equal(t, "somebody else\n"+keyLine, read(t, path))
 
-	// The fetch and nothing after it: the tool did not connect again.
-	require.Len(t, recorded(t, pretend.batch), 1)
+	// The read and nothing after it: the tool did not connect again.
+	require.Equal(t, 1, connections(t, pretend))
 }
 
 func TestAnEmptyFileGetsTheKeyAndNoBlankLineBeforeIt(t *testing.T) {
@@ -218,7 +240,9 @@ func TestTheFileIsUploadedBesideTheOldOneAndThenRenamedOverIt(t *testing.T) {
 		strings.HasPrefix(beside, ".ssh/authorized_keys.keyfunc-"),
 	)
 
-	require.True(t, strings.HasPrefix(sent[0], "get .ssh/authorized_keys "))
+	// The listing fails on a host with no .ssh, so the get never runs;
+	// the write session then makes the directory and puts the file.
+	require.Equal(t, "ls -1 .ssh", sent[0])
 	require.Equal(t, "-mkdir .ssh", sent[1])
 	require.Equal(t, "chmod 700 .ssh", sent[2])
 	require.Equal(t, "put", strings.Fields(sent[3])[0])
@@ -237,12 +261,57 @@ func TestAFileThatCannotBeReadIsNotWrittenOver(t *testing.T) {
 	require.Empty(t, printed)
 	require.Contains(t, said, "Permission denied")
 
-	// The fetch and nothing after it, and what was on the host is
+	// The read and nothing after it, and what was on the host is
 	// still what is on the host.
-	require.Len(t, recorded(t, pretend.batch), 1)
+	require.Equal(t, 1, connections(t, pretend))
 	require.DirExists(t, unreadable)
 }
 
+func TestAnUnreadableDirectoryIsNotWrittenInto(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+	unlistable(t, pretend)
+
+	// The listing is refused, which is not the same as no directory, so
+	// the tool writes nothing rather than treat a directory it cannot
+	// enter as a host with no file.
+	printed, said, err := attempt(t, host)
+	require.Error(t, err)
+	require.Empty(t, printed)
+	require.Contains(t, said, "Permission denied")
+
+	// The read and nothing after it: no second connection wrote a key.
+	require.Equal(t, 1, connections(t, pretend))
+}
+
+func TestAnExistingDirectoryKeepsItsModeAndIsNotRemade(t *testing.T) {
+	t.Setenv(mnemonic.Variable, example())
+
+	pretend := pretendHost(t)
+
+	// A directory that is there but holds no file yet, made with a mode
+	// of its own so that a stray chmod would show.
+	const ownMode = 0o755
+
+	directory := filepath.Join(pretend.home, keptUnder)
+	require.NoError(t, os.Mkdir(directory, ownMode))
+
+	require.Equal(t, "added\n", install(t, host))
+
+	// The key is added and the directory keeps the mode it had: the
+	// write session neither made it nor set its mode.
+	require.Equal(t, keyLine, read(t, filepath.Join(directory, keptIn)))
+
+	kept, err := os.Stat(directory)
+	require.NoError(t, err)
+	require.Equal(t, os.FileMode(ownMode), kept.Mode().Perm())
+
+	sent := recorded(t, pretend.batch)
+	require.NotContains(t, sent, "-mkdir .ssh")
+	require.NotContains(t, sent, "chmod 700 .ssh")
+}
+
 func TestAWarningAboutAnotherFileIsNotTakenForTheOneAskedFor(t *testing.T) {
 	t.Setenv(mnemonic.Variable, example())
 
@@ -259,7 +328,7 @@ func TestAWarningAboutAnotherFileIsNotTakenForTheOneAskedFor(t *testing.T) {
 	require.Contains(t, said, "No such file or directory")
 	require.Contains(t, said, "Permission denied")
 
-	require.Len(t, recorded(t, pretend.batch), 1)
+	require.Equal(t, 1, connections(t, pretend))
 	require.DirExists(t, unreadable)
 
 	// The same run again, this way for the status it ends with.
@@ -279,9 +348,9 @@ func TestAFailedStepNamesTheUploadedFileAndChangesNothing(t *testing.T) {
 
 	pretend := pretendHost(t)
 
-	// A file where the .ssh directory belongs: nothing is there to
-	// fetch, and then the put has nowhere to put anything, so the
-	// write session ends at the put.
+	// A file where the .ssh directory belongs: the listing shows it and
+	// so the directory reads as already there, but then the put has
+	// nowhere to put anything, so the write session ends at the put.
 	inTheWay := filepath.Join(pretend.home, keptUnder)
 	require.NoError(t,
 		os.WriteFile(inTheWay, []byte(notADirectory), fileMode),
@@ -292,12 +361,12 @@ func TestAFailedStepNamesTheUploadedFileAndChangesNothing(t *testing.T) {
 	require.Empty(t, printed)
 	require.Contains(t, said, "put failed")
 
-	// The put is the last command the session got to, and the file it
-	// was uploading is the one the message names.
+	// The put is the first and last command the write session got to,
+	// and the file it was uploading is the one the message names.
 	sent := recorded(t, pretend.batch)
-	require.Len(t, sent, 4)
-	require.Equal(t, "put", strings.Fields(sent[3])[0])
-	require.Contains(t, err.Error(), strings.Fields(sent[3])[2])
+	require.Len(t, sent, 3)
+	require.Equal(t, "put", strings.Fields(sent[2])[0])
+	require.Contains(t, err.Error(), strings.Fields(sent[2])[2])
 
 	require.Equal(t, notADirectory, read(t, inTheWay))
 
@@ -455,6 +524,38 @@ func unfetchable(t *testing.T, pretend pretended) string {
 	return path
 }
 
+// unlistable puts a .ssh on the stand-in host that is there but shut to
+// the user, a directory of mode 000, and gives back its path. Its mode
+// is put back before the temporary directory is cleared so that it can
+// be.
+func unlistable(t *testing.T, pretend pretended) string {
+	t.Helper()
+
+	directory := filepath.Join(pretend.home, keptUnder)
+	require.NoError(t, os.Mkdir(directory, directoryMode))
+	require.NoError(t, os.Chmod(directory, 0))
+
+	t.Cleanup(func() { _ = os.Chmod(directory, directoryMode) })
+
+	return directory
+}
+
+// connections returns how many times the tool ran sftp, counted from
+// the -b that opens each session's arguments.
+func connections(t *testing.T, pretend pretended) int {
+	t.Helper()
+
+	count := 0
+
+	for _, argument := range recorded(t, pretend.arguments) {
+		if argument == "-b" {
+			count++
+		}
+	}
+
+	return count
+}
+
 // pretendCall puts the to stand-in on the path and gives back the file
 // the arguments are written down in and the file the agent socket is
 // noted in.