unit: stable serialization order

6654289 introduced consolidation of
UnitOptions into Sections during serialization. This preserved ordering
of individual options within sections, but since a map was used to house
the sections their order was nondeterministic.

This change moves to preserving the first-seen order of the sections, as
well as continuing to preserve the order of units with sections. For
example:

	[UnitOption{A, B, b}, UnitOption{C, D, d}, UnitOption{A, Z, z}]

will consistently be written as:

	[A]
	B=b
	Z=z
	[C]
	D=d

unit/serialize.go

@@ -19,28 +19,40 @@ import (
 	"io"
 )
 
-// Serialize encodes all of the given UnitOption objects into a unit file
+// Serialize encodes all of the given UnitOption objects into a
+// unit file. When serialized the options are sorted in their
+// supplied order but grouped by section.
 func Serialize(opts []*UnitOption) io.Reader {
 	var buf bytes.Buffer
 
 	if len(opts) == 0 {
 		return &buf
 	}
 
+	// Index of sections -> ordered options
 	idx := map[string][]*UnitOption{}
+	// Separately preserve order in which sections were seen
+	sections := []string{}
 	for _, opt := range opts {
-		idx[opt.Section] = append(idx[opt.Section], opt)
+		sec := opt.Section
+		if _, ok := idx[sec]; !ok {
+			sections = append(sections, sec)
+		}
+		idx[sec] = append(idx[sec], opt)
 	}
 
-	for curSection, curOpts := range idx {
-		writeSectionHeader(&buf, curSection)
+	for i, sect := range sections {
+		writeSectionHeader(&buf, sect)
 		writeNewline(&buf)
 
-		for _, opt := range curOpts {
+		opts := idx[sect]
+		for _, opt := range opts {
 			writeOption(&buf, opt)
 			writeNewline(&buf)
 		}
-		writeNewline(&buf)
+		if i < len(sections)-1 {
+			writeNewline(&buf)
+		}
 	}
 
 	return &buf

unit/serialize_test.go

@@ -68,7 +68,7 @@ ExecStart=/usr/bin/sleep infinity
 `,
 		},
 
-		// no optimization for unsorted options
+		// options are grouped into sections
 		{
 			[]*UnitOption{
 				&UnitOption{"Unit", "Description", "Foo"},
@@ -77,12 +77,34 @@ ExecStart=/usr/bin/sleep infinity
 			},
 			`[Unit]
 Description=Foo
+BindsTo=bar.service
 
 [Service]
 ExecStart=/usr/bin/sleep infinity
+`,
+		},
 
-[Unit]
+		// options are ordered within groups, and sections are ordered in the order in which they were first seen
+		{
+			[]*UnitOption{
+				&UnitOption{"Unit", "Description", "Foo"},
+				&UnitOption{"Service", "ExecStart", "/usr/bin/sleep infinity"},
+				&UnitOption{"Unit", "BindsTo", "bar.service"},
+				&UnitOption{"X-Foo", "Bar", "baz"},
+				&UnitOption{"Service", "ExecStop", "/usr/bin/sleep 1"},
+				&UnitOption{"Unit", "Documentation", "https://foo.com"},
+			},
+			`[Unit]
+Description=Foo
 BindsTo=bar.service
+Documentation=https://foo.com
+
+[Service]
+ExecStart=/usr/bin/sleep infinity
+ExecStop=/usr/bin/sleep 1
+
+[X-Foo]
+Bar=baz
 `,
 		},
 
@@ -140,7 +162,7 @@ o
 
 		output := string(outBytes)
 		if tt.output != output {
-			t.Errorf("case %d: incorrect output")
+			t.Errorf("case %d: incorrect output", i)
 			t.Logf("Expected:\n%s", tt.output)
 			t.Logf("Actual:\n%s", output)
 		}