Support Acquire-By-Hash for index files

The added "aptly publish repo" option "-access-by-hash" publishes
the index files (Packages*, Sources*) also as hardlinked hashes.
Example:
 /dists/yakkety/main/binary-amd64/by-hash/SHA512/31833ec39acc...
The Release files indicate this with the option "Acquire-By-Hash: yes"

This is used by apt >= 1.2.0 and prevents the "Hash sum mismatch" race
condition between a server side "aptly publish repo" and "apt-get update"
on a client.
See: http://www.chiark.greenend.org.uk/~cjwatson/blog/no-more-hash-sum-mismatch-errors.html

This implementation uses symlinks in the by-hash/*/ directory for keeping
only two versions of the index files and deleting older files
automatically.

Note: this only works with aptly.FileSystemPublishedStorage

Closes: #536

Signed-off-by: André Roth <neolynx@gmail.com>

deb/publish.go

@@ -64,6 +64,9 @@ type PublishedRepo struct {
 
 	// True if repo is being re-published
 	rePublishing bool
+
+	// Provide index files per hash also
+	AccessByHash bool
 }
 
 // ParsePrefix splits [storage:]prefix into components
@@ -556,7 +559,7 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 	}
 	defer os.RemoveAll(tempDir)
 
-	indexes := newIndexFiles(publishedStorage, basePath, tempDir, suffix)
+	indexes := newIndexFiles(publishedStorage, basePath, tempDir, suffix, p.AccessByHash)
 
 	for component, list := range lists {
 		hadUdebs := false
@@ -683,6 +686,9 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 				release["Component"] = component
 				release["Origin"] = p.GetOrigin()
 				release["Label"] = p.GetLabel()
+				if p.AccessByHash {
+					release["Acquire-By-Hash"] = "yes"
+				}
 
 				var bufWriter *bufio.Writer
 				bufWriter, err = indexes.ReleaseIndex(component, arch, udeb).BufWriter()
@@ -720,6 +726,9 @@ func (p *PublishedRepo) Publish(packagePool aptly.PackagePool, publishedStorageP
 	release["Codename"] = p.Distribution
 	release["Date"] = time.Now().UTC().Format("Mon, 2 Jan 2006 15:04:05 MST")
 	release["Architectures"] = strings.Join(utils.StrSlicesSubstract(p.Architectures, []string{ArchitectureSource}), " ")
+	if p.AccessByHash {
+		release["Acquire-By-Hash"] = "yes"
+	}
 	release["Description"] = " Generated by aptly\n"
 	release["MD5Sum"] = ""
 	release["SHA1"] = ""