quadlet: Support systemd style dropin files

For a source file like `foo.container`, look for drop in named
`foo.container.d/*.conf` and merged them into the main file.  The
dropins are applied in alphabetical order, and files in earlier
diretories override later files with same name.

This is similar to how systemd dropins work, see:
https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html

Also adds some tests for these

Signed-off-by: Alexander Larsson <alexl@redhat.com>

cmd/quadlet/main.go

@@ -242,6 +242,67 @@ func loadUnitsFromDir(sourcePath string) ([]*parser.UnitFile, error) {
 	return units, prevError
 }
 
+func loadUnitDropins(unit *parser.UnitFile, sourcePaths []string) error {
+	var prevError error
+	reportError := func(err error) {
+		if prevError != nil {
+			err = fmt.Errorf("%s\n%s", prevError, err)
+		}
+		prevError = err
+	}
+
+	var dropinPaths = make(map[string]string)
+	for _, sourcePath := range sourcePaths {
+		dropinDir := path.Join(sourcePath, unit.Filename+".d")
+
+		dropinFiles, err := os.ReadDir(dropinDir)
+		if err != nil {
+			if !errors.Is(err, os.ErrNotExist) {
+				reportError(fmt.Errorf("error reading directory %q, %w", dropinDir, err))
+			}
+
+			continue
+		}
+
+		for _, dropinFile := range dropinFiles {
+			dropinName := dropinFile.Name()
+			if filepath.Ext(dropinName) != ".conf" {
+				continue // Only *.conf supported
+			}
+
+			if _, ok := dropinPaths[dropinName]; ok {
+				continue // We already saw this name
+			}
+
+			dropinPaths[dropinName] = path.Join(dropinDir, dropinName)
+		}
+	}
+
+	dropinFiles := make([]string, len(dropinPaths))
+	i := 0
+	for k := range dropinPaths {
+		dropinFiles[i] = k
+		i++
+	}
+
+	// Merge in alpha-numerical order
+	sort.Strings(dropinFiles)
+
+	for _, dropinFile := range dropinFiles {
+		dropinPath := dropinPaths[dropinFile]
+
+		Debugf("Loading source drop-in file %s", dropinPath)
+
+		if f, err := parser.ParseUnitFile(dropinPath); err != nil {
+			reportError(fmt.Errorf("error loading %q, %w", dropinPath, err))
+		} else {
+			unit.Merge(f)
+		}
+	}
+
+	return prevError
+}
+
 func generateServiceFile(service *parser.UnitFile) error {
 	Debugf("writing %q", service.Path)
 
@@ -456,6 +517,12 @@ func process() error {
 		return prevError
 	}
 
+	for _, unit := range units {
+		if err := loadUnitDropins(unit, sourcePaths); err != nil {
+			reportError(err)
+		}
+	}
+
 	if !dryRunFlag {
 		err := os.MkdirAll(outputPath, os.ModePerm)
 		if err != nil {

docs/source/markdown/podman-systemd.unit.5.md

@@ -47,6 +47,13 @@ Each file type has a custom section (for example, `[Container]`) that is handled
 other sections are passed on untouched, allowing the use of any normal systemd configuration options
 like dependencies or cgroup limits.
 
+The source files also support drop-ins in the same [way systemd does](https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html).
+For a given source file (say `foo.container`), the corresponding `.d`directory (in this
+case `foo.container.d`) will be scanned for files with a `.conf` extension that are merged into
+the base file in alphabetical order. The format of these drop-in files is the same as the base file.
+This is useful to alter or add configuration settings for a unit, without having to modify unit
+files.
+
 For rootless containers, when administrators place Quadlet files in the
 /etc/containers/systemd/users directory, all users' sessions execute the
 Quadlet when the login session begins. If the administrator places a Quadlet

pkg/systemd/parser/unitfile.go

@@ -182,7 +182,7 @@ func (f *UnitFile) ensureGroup(groupName string) *unitGroup {
 	return g
 }
 
-func (f *UnitFile) merge(source *UnitFile) {
+func (f *UnitFile) Merge(source *UnitFile) {
 	for _, srcGroup := range source.groups {
 		group := f.ensureGroup(srcGroup.name)
 		group.merge(srcGroup)
@@ -193,7 +193,7 @@ func (f *UnitFile) merge(source *UnitFile) {
 func (f *UnitFile) Dup() *UnitFile {
 	copy := NewUnitFile()
 
-	copy.merge(f)
+	copy.Merge(f)
 	copy.Filename = f.Filename
 	return copy
 }

test/e2e/quadlet/merged-override.container

@@ -0,0 +1,8 @@
+## assert-podman-final-args localhost/imagename
+## !assert-podman-args --env "MAIN=mainvalue"
+## !assert-podman-args --env "FIRST=value"
+## assert-podman-args --env "SECOND=othervalue"
+
+[Container]
+Image=localhost/imagename
+Environment=MAIN=mainvalue

test/e2e/quadlet/merged-override.container.d/10-first.conf

@@ -0,0 +1,2 @@
+[Container]
+Environment=FIRST=value

test/e2e/quadlet/merged-override.container.d/20-second.conf

@@ -0,0 +1,4 @@
+[Container]
+# Empty previous
+Environment=
+Environment=SECOND=othervalue

test/e2e/quadlet/merged.container

@@ -0,0 +1,8 @@
+## assert-podman-final-args localhost/imagename
+## assert-podman-args --env "MAIN=mainvalue"
+## assert-podman-args --env "FIRST=value"
+## assert-podman-args --env "SECOND=othervalue"
+
+[Container]
+Image=localhost/imagename
+Environment=MAIN=mainvalue

test/e2e/quadlet/merged.container.d/10-first.conf

@@ -0,0 +1,2 @@
+[Container]
+Environment=FIRST=value

test/e2e/quadlet/merged.container.d/20-second.conf

@@ -0,0 +1,2 @@
+[Container]
+Environment=SECOND=othervalue

test/e2e/quadlet_test.go

@@ -664,6 +664,16 @@ BOGUS=foo
 			err = os.WriteFile(filepath.Join(quadletDir, fileName), testcase.data, 0644)
 			Expect(err).ToNot(HaveOccurred())
 
+			// Also copy any extra snippets
+			dotdDir := filepath.Join("quadlet", fileName+".d")
+			if s, err := os.Stat(dotdDir); err == nil && s.IsDir() {
+				dotdDirDest := filepath.Join(quadletDir, fileName+".d")
+				err = os.Mkdir(dotdDirDest, os.ModePerm)
+				Expect(err).ToNot(HaveOccurred())
+				err = CopyDirectory(dotdDir, dotdDirDest)
+				Expect(err).ToNot(HaveOccurred())
+			}
+
 			// Run quadlet to convert the file
 			session := podmanTest.Quadlet([]string{"--user", "--no-kmsg-log", generatedDir}, quadletDir)
 			session.WaitWithDefaultTimeout()
@@ -748,6 +758,8 @@ BOGUS=foo
 		Entry("workingdir.container", "workingdir.container", 0, ""),
 		Entry("Container - global args", "globalargs.container", 0, ""),
 		Entry("Container - Containers Conf Modules", "containersconfmodule.container", 0, ""),
+		Entry("merged.container", "merged.container", 0, ""),
+		Entry("merged-override.container", "merged-override.container", 0, ""),
 
 		Entry("basic.volume", "basic.volume", 0, ""),
 		Entry("device-copy.volume", "device-copy.volume", 0, ""),