Enforce clang-format version 7.x

Summary:
`llvm` 8.0 and 7.1 have been released recently.

As an example, Ubuntu recently updated to clang-format 7.1, causing
arcanist to return an error on `arc lint` due to version conflict.

I tested our codebase against clang-format 7.1 and 8.0, there is
a minor difference in the output (some edge cases are causing
bad indents with clang-format-7, which is fixed by clang-format-8).

Therefore both versions cannot be allowed at the same time
as it would cause `arc lint` to format back and forth when
using different versions.

I would have preferred to enforce >= 7.0 and < 8 in the
`.arclint`file but this is not supported by arcanist.
As a workaround an additional check is performed at the linter
level to enforce using version 7.x only.

Test Plan:
With `clang-format` 7.0, 7.1 and 8.0:
  arc lint --everything

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3806

.arclint

@@ -5,7 +5,7 @@
         },
         "clang-format": {
             "type": "clang-format",
-            "version": "7.0",
+            "version": ">=7.0",
             "bin": ["clang-format-7", "clang-format"],
             "include": "(^src/.*\\.(h|c|cpp|mm)$)",
             "exclude": [

arcanist/linter/ClangFormatLinter.php

@@ -40,10 +40,25 @@ public function getVersion() {
     $matches = array();
     $regex = '/^clang-format version (?P<version>\d+\.\d+)\./';
     if (preg_match($regex, $stdout, $matches)) {
-      return $matches['version'];
+      $version = $matches['version'];
     } else {
       return false;
     }
+
+    /*
+     * FIXME: This is a hack to only allow for clang-format version 7.x.
+     * The .arclint `version` field only allow to filter versions using `=`,
+     * `>`, `<`, `>=` or `<=`. There is no facility to define that the required
+     * version should be >= 7.0 and < 8.0.
+     */
+    if ($version[0] != '7') {
+      throw new Exception(pht('Linter %s requires clang-format version 7.x. '.
+                              'You have version %s.',
+                              ClangFormatLinter::class,
+                              $version));
+    }
+
+    return $version;
   }
 
   public function getInstallInstructions() {

arcanist/phpcs.xml

@@ -17,6 +17,8 @@
         <exclude name="Generic.Commenting.DocComment.ParamNotFirst" />
         <exclude name="Generic.Commenting.DocComment.SpacingBeforeTags" />
         <exclude name="Generic.Commenting.DocComment.TagValueIndent" />
+        <exclude name="Generic.Commenting.Fixme.TaskFound" />
+        <exclude name="Generic.Commenting.Todo.TaskFound" />
         <exclude name="Generic.Files.EndFileNoNewline.Found" />
         <exclude name="Generic.Files.LowercasedFilename" />
         <exclude name="Generic.Formatting.MultipleStatementAlignment" />