diff --git a/doc/03-cli.md b/doc/03-cli.md
index d5064c5d9..49879d54d 100644
--- a/doc/03-cli.md
+++ b/doc/03-cli.md
@@ -920,6 +920,10 @@ If you use a proxy but it does not support the request_fulluri flag for HTTPS
 requests, then you should set this env var to `false` or `0` to prevent Composer
 from setting the request_fulluri option.
 
+### COMPOSER_SELF_UPDATE_TARGET
+
+If set, makes the self-update command write the new Composer phar file into that path instead of overwriting itself. Useful for updating Composer on read-only filesystem.
+
 ### no_proxy or NO_PROXY
 
 If you are behind a proxy and would like to disable it for certain domains, you
diff --git a/src/Composer/Command/SelfUpdateCommand.php b/src/Composer/Command/SelfUpdateCommand.php
index 78b27460e..226aafeb5 100644
--- a/src/Composer/Command/SelfUpdateCommand.php
+++ b/src/Composer/Command/SelfUpdateCommand.php
@@ -351,6 +351,10 @@ TAGSPUBKEY
                 @copy($localFilename, $backupTarget);
             }
 
+            if ($targetFilename = getenv('COMPOSER_SELF_UPDATE_TARGET')) {
+                $localFilename = realpath($targetFilename) ?: $targetFilename;
+            }
+
             rename($newFilename, $localFilename);
 
             return null;
diff --git a/src/Composer/DependencyResolver/Problem.php b/src/Composer/DependencyResolver/Problem.php
index bb497b549..df289a4c1 100644
--- a/src/Composer/DependencyResolver/Problem.php
+++ b/src/Composer/DependencyResolver/Problem.php
@@ -108,7 +108,7 @@ class Problem
 
                     $msg = "\n    - This package requires ".$packageName.$this->constraintToText($constraint).' but ';
 
-                    if (defined('HHVM_VERSION') || count($available)) {
+                    if (defined('HHVM_VERSION') || (count($available) && $packageName === 'hhvm')) {
                         return $msg . 'your HHVM version does not satisfy that requirement.';
                     }
 
diff --git a/src/Composer/Package/AliasPackage.php b/src/Composer/Package/AliasPackage.php
index 09ed4fb9b..89f197856 100644
--- a/src/Composer/Package/AliasPackage.php
+++ b/src/Composer/Package/AliasPackage.php
@@ -401,4 +401,14 @@ class AliasPackage extends BasePackage implements CompletePackageInterface
     {
         return parent::__toString().' (alias of '.$this->aliasOf->getVersion().')';
     }
+
+    public function setDistUrl($url)
+    {
+        return $this->aliasOf->setDistUrl($url);
+    }
+
+    public function setDistType($type)
+    {
+        return $this->aliasOf->setDistType($type);
+    }
 }
diff --git a/src/Composer/Package/PackageInterface.php b/src/Composer/Package/PackageInterface.php
index 73d2ade41..cb16efa7e 100644
--- a/src/Composer/Package/PackageInterface.php
+++ b/src/Composer/Package/PackageInterface.php
@@ -358,4 +358,32 @@ interface PackageInterface
      * @return array
      */
     public function getTransportOptions();
+
+    /**
+     * @param string $reference
+     *
+     * @return void
+     */
+    public function setSourceReference($reference);
+
+    /**
+     * @param string $url
+     *
+     * @return void
+     */
+    public function setDistUrl($url);
+
+    /**
+     * @param string $type
+     *
+     * @return void
+     */
+    public function setDistType($type);
+
+    /**
+     * @param string $reference
+     *
+     * @return void
+     */
+    public function setDistReference($reference);
 }
diff --git a/src/Composer/Repository/ComposerRepository.php b/src/Composer/Repository/ComposerRepository.php
index 38b865103..9d5b727cc 100644
--- a/src/Composer/Repository/ComposerRepository.php
+++ b/src/Composer/Repository/ComposerRepository.php
@@ -562,7 +562,11 @@ class ComposerRepository extends ArrayRepository implements ConfigurableReposito
     protected function canonicalizeUrl($url)
     {
         if ('/' === $url[0]) {
-            return preg_replace('{(https?://[^/]+).*}i', '$1' . $url, $this->url);
+            if (preg_match('{^[^:]++://[^/]*+}', $this->url, $matches)) {
+                return $matches[0] . $url;
+            }
+
+            return $this->url;
         }
 
         return $url;
diff --git a/tests/Composer/Test/Repository/ComposerRepositoryTest.php b/tests/Composer/Test/Repository/ComposerRepositoryTest.php
index 3e29e8023..8e9216b35 100644
--- a/tests/Composer/Test/Repository/ComposerRepositoryTest.php
+++ b/tests/Composer/Test/Repository/ComposerRepositoryTest.php
@@ -204,4 +204,71 @@ class ComposerRepositoryTest extends TestCase
             $repository->search('foo', RepositoryInterface::SEARCH_FULLTEXT, 'library')
         );
     }
+
+    /**
+     * @dataProvider canonicalizeUrlProvider
+     *
+     * @param string $expected
+     * @param string $url
+     * @param string $repositoryUrl
+     */
+    public function testCanonicalizeUrl($expected, $url, $repositoryUrl)
+    {
+        $repository = new ComposerRepository(
+            array('url' => $repositoryUrl),
+            new NullIO(),
+            FactoryMock::createConfig()
+        );
+
+        $object = new \ReflectionObject($repository);
+
+        $method = $object->getMethod('canonicalizeUrl');
+        $method->setAccessible(true);
+
+        // ComposerRepository::__construct ensures that the repository URL has a
+        // protocol, so reset it here in order to test all cases.
+        $property = $object->getProperty('url');
+        $property->setAccessible(true);
+        $property->setValue($repository, $repositoryUrl);
+
+        $this->assertSame($expected, $method->invoke($repository, $url));
+    }
+
+    public function canonicalizeUrlProvider()
+    {
+        return array(
+            array(
+                'https://example.org/path/to/file',
+                '/path/to/file',
+                'https://example.org',
+            ),
+            array(
+                'https://example.org/canonic_url',
+                'https://example.org/canonic_url',
+                'https://should-not-see-me.test',
+            ),
+            array(
+                'file:///path/to/repository/file',
+                '/path/to/repository/file',
+                'file:///path/to/repository',
+            ),
+            array(
+                // Assert that the repository URL is returned unchanged if it is
+                // not a URL.
+                // (Backward compatibility test)
+                'invalid_repo_url',
+                '/path/to/file',
+                'invalid_repo_url',
+            ),
+            array(
+                // Assert that URLs can contain sequences resembling pattern
+                // references as understood by preg_replace() without messing up
+                // the result.
+                // (Regression test)
+                'https://example.org/path/to/unusual_$0_filename',
+                '/path/to/unusual_$0_filename',
+                'https://example.org',
+            ),
+        );
+    }
 }