BinaryInstaller: install full binaries on WSL when bin-compat=auto (#9855)

src/Composer/Installer/BinaryInstaller.php

@@ -82,7 +82,7 @@ class BinaryInstaller
             }
 
             if ($this->binCompat === "auto") {
-                if (Platform::isWindows()) {
+                if (Platform::isWindows() || Platform::isWindowsSubsystemForLinux()) {
                     $this->installFullBinaries($binPath, $link, $bin, $package);
                 } else {
                     $this->installSymlinkBinaries($binPath, $link);

src/Composer/Util/Platform.php

@@ -21,6 +21,8 @@ class Platform
 {
     /** @var ?bool */
     private static $isVirtualBoxGuest = null;
+    /** @var ?bool */
+    private static $isWindowsSubsystemForLinux = null;
 
     /**
      * Parses tildes and environment variables in paths.
@@ -67,6 +69,27 @@ class Platform
         throw new \RuntimeException('Could not determine user directory');
     }
 
+    /**
+     * @return bool Whether the host machine is running on the Windows Subsystem for Linux (WSL)
+     */
+    public static function isWindowsSubsystemForLinux()
+    {
+        if (null === self::$isWindowsSubsystemForLinux) {
+            self::$isWindowsSubsystemForLinux = false;
+            
+            // while WSL will be hosted within windows, WSL itself cannot be windows based itself.
+            if (self::isWindows()) {
+                return self::$isWindowsSubsystemForLinux = false;
+            }
+
+            if (is_readable('/proc/version') && false !== stripos(file_get_contents('/proc/version'), 'microsoft')) {
+                return self::$isWindowsSubsystemForLinux = true;
+            }
+        }
+
+        return self::$isWindowsSubsystemForLinux;
+    }
+
     /**
      * @return bool Whether the host machine is running a Windows OS
      */