ssh/config: refactoring the Config merge

This changes rename method [Config.Prepend] to [Config.Merge].

The way that how the other Config merged is changed.
Instead of appending all of other's sections into the current Config,
append the other Config instance to the current instance of Config.

During [Config.Get] the top Config will be evaluated first, and then the
other Config is evaluated in order of Merge.

lib/ssh/config/config.go

@@ -40,6 +40,9 @@ type Config struct {
 	homeDir string
 
 	sections []*Section
+
+	// others Config, as result of [Config.Merge].
+	others []*Config
 }
 
 // newConfig create new SSH Config instance from file.
@@ -156,6 +159,16 @@ func (cfg *Config) Get(host string) (section *Section) {
 			section.merge(hostMatch)
 		}
 	}
+
+	var (
+		other  *Config
+		subsec *Section
+	)
+	for _, other = range cfg.others {
+		subsec = other.Get(host)
+		section.merge(subsec)
+	}
+
 	section.setDefaults()
 
 	if host != `` && section.Field[KeyHostname] == `` {
@@ -165,16 +178,18 @@ func (cfg *Config) Get(host string) (section *Section) {
 	return section
 }
 
-// Prepend other Config's sections to this Config.
-// The other's sections will be at the top of the list.
+// Merge other Config as part of this Config.
+// This function can be used to combine multiple SSH config files into one.
 //
-// This function can be useful if we want to load another SSH config file
-// without using Include directive.
-func (cfg *Config) Prepend(other *Config) {
-	newSections := make([]*Section, 0, len(cfg.sections)+len(other.sections))
-	newSections = append(newSections, other.sections...)
-	newSections = append(newSections, cfg.sections...)
-	cfg.sections = newSections
+// For example after the user's "~/.ssh/config" has been loaded, we can
+// merge it with system "/etc/ssh/ssh_config".
+// During [Config.Get] the top Config will be evaluated first, and then the
+// other Config is evaluated in order of Merge-d.
+func (cfg *Config) Merge(other *Config) {
+	if other == nil {
+		return
+	}
+	cfg.others = append(cfg.others, other)
 }
 
 // loadEnvironments get all environments variables and store it in the map for

lib/ssh/config/config_test.go

@@ -115,6 +115,45 @@ func TestConfigGet(t *testing.T) {
 	}
 }
 
+func TestConfigMerge(t *testing.T) {
+	var (
+		tdata *test.Data
+		err   error
+	)
+
+	tdata, err = test.LoadData(`testdata/config_merge_test.txt`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var cfg *Config
+
+	cfg, err = Load(`./testdata/sub/config`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var topcfg *Config
+
+	topcfg, err = Load(`./testdata/config`)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cfg.Merge(topcfg)
+
+	var (
+		host       = `my.example.local`
+		gotSection = cfg.Get(host)
+
+		buf bytes.Buffer
+	)
+
+	gotSection.WriteTo(&buf)
+
+	test.Assert(t, host, string(tdata.Output[host]), buf.String())
+}
+
 func TestParseKeyValue(t *testing.T) {
 	cases := []struct {
 		line     string

lib/ssh/config/parser_test.go

@@ -75,7 +75,7 @@ func TestReadLines(t *testing.T) {
 			`User test`,
 			`IdentityFile ~/.ssh/notexist`,
 			`Host *.example.local`,
-			`Include sub/config`,
+			`Include sub/include`,
 			`Host foo.local`,
 			`Hostname 127.0.0.3`,
 			`Port 28022`,
@@ -123,6 +123,7 @@ func TestConfigParser_load(t *testing.T) {
 			`Hostname 127.0.0.2`,
 			`User wildcard`,
 			`IdentityFile ~/.ssh/notexist`,
+			`UserKnownHostsFile ~/.ssh/known_hosts_example_local`,
 			`Host foo.local`,
 			`Hostname 127.0.0.3`,
 			`Port 28022`,

lib/ssh/config/section.go

@@ -667,6 +667,16 @@ func (section *Section) MarshalText() (text []byte, err error) {
 			}
 			continue
 		}
+		if key == KeyUserKnownHostsFile {
+			for _, val = range section.knownHostsFile {
+				buf.WriteString(`  `)
+				buf.WriteString(key)
+				buf.WriteByte(' ')
+				buf.WriteString(section.pathFold(val))
+				buf.WriteByte('\n')
+			}
+			continue
+		}
 
 		buf.WriteString(`  `)
 		buf.WriteString(key)
@@ -688,13 +698,18 @@ func (section *Section) WriteTo(w io.Writer) (n int64, err error) {
 	return int64(c), err
 }
 
-// pathFold replace the home directory prefix with '~'.
+// pathFold remove the path prefix from input file "in", start from the
+// "config" directory and then the user's home directory.
 func (section *Section) pathFold(in string) (out string) {
-	if !strings.HasPrefix(in, section.homeDir) {
-		return in
+	if filepath.HasPrefix(in, section.dir) {
+		out, _ = filepath.Rel(section.dir, in)
+		return out
+	}
+	if filepath.HasPrefix(in, section.homeDir) {
+		out, _ = filepath.Rel(section.homeDir, in)
+		return `~/` + out
 	}
-	out = `~` + in[len(section.homeDir):]
-	return out
+	return in
 }
 
 // pathUnfold expand the file to make it absolute.

lib/ssh/config/testdata/config

@@ -8,7 +8,7 @@ Host example.local
 
 	# comment
 Host *.example.local
-	Include sub/config
+	Include sub/include
 
 Host foo.local
 	Hostname 127.0.0.3

lib/ssh/config/testdata/config_get_test.txt

@@ -28,6 +28,7 @@ Host my.example.local
   identityfile ~/.ssh/notexist
   port 22
   user wildcard
+  userknownhostsfile ~/.ssh/known_hosts_example_local
   xauthlocation /usr/X11R6/bin/xauth
 
 <<< foo.local

lib/ssh/config/testdata/config_merge_test.txt

@@ -0,0 +1,15 @@
+Test data for [Config.Merge].
+
+<<< my.example.local
+Host my.example.local
+  challengeresponseauthentication yes
+  checkhostip yes
+  connectionattempts 1
+  hostname 127.0.0.2
+  identityfile my-example-local
+  identityfile ~/.ssh/notexist
+  port 22
+  user wildcard
+  userknownhostsfile my_known_hosts
+  userknownhostsfile ~/.ssh/known_hosts_example_local
+  xauthlocation /usr/X11R6/bin/xauth

lib/ssh/config/testdata/sub/config

@@ -1,3 +1,3 @@
-	Hostname 127.0.0.2
-	User wildcard
-	IdentityFile ~/.ssh/notexist
+Host my.example.local
+  UserKnownHostsFile my_known_hosts
+  IdentityFile my-example-local

lib/ssh/config/testdata/sub/include

@@ -0,0 +1,4 @@
+  Hostname 127.0.0.2
+  User wildcard
+  IdentityFile ~/.ssh/notexist
+  UserKnownHostsFile ~/.ssh/known_hosts_example_local