Fix support for read-only caches where the filesystem is not writable (fixes #10906)

CHANGELOG.md

@@ -1,5 +1,6 @@
 ### [2.2.15] 2022-07-01
 
+  * Fixed support for read-only caches where the filesystem is not writable (#10906)
   * Fixed type error when using `allow-plugins: true` (#10909)
   * Fixed @putenv scripts receiving arguments passed to the command (#10846)
   * Fixed support for spaces in paths with binary proxies on Windows (#10836)

src/Composer/Cache.php

@@ -98,8 +98,11 @@ class Cache
             $this->enabled = true;
 
             if (
-                (!is_dir($this->root) && !Silencer::call('mkdir', $this->root, 0777, true))
-                || !is_writable($this->root)
+                !$this->readOnly
+                && (
+                    (!is_dir($this->root) && !Silencer::call('mkdir', $this->root, 0777, true))
+                    || !is_writable($this->root)
+                )
             ) {
                 $this->io->writeError('<warning>Cannot create cache directory ' . $this->root . ', or directory is not writable. Proceeding without cache</warning>');
                 $this->enabled = false;