Merge pull request swiftlang#123 from apple/multicommit2

 Support building multiple pinned version commits

Author: lplarson

README.md

@@ -70,11 +70,12 @@ To add a new Swift Package Manager project, use the following template:
   "path": "project",
   "branch": "master",
   "maintainer": "[email protected]",
-  "compatibility": {
-    "3.0": {
+  "compatibility": [
+    {
+      "version": "3.0",
       "commit": "195cd8cde2bb717242b3081f9c367ccd0a2f0121"
     }
-  },
+  ],
   "platforms": [
     "Darwin"
   ],
@@ -90,10 +91,13 @@ To add a new Swift Package Manager project, use the following template:
 }
 ~~~
 
-The `commit` field specifies a commit hash to pin the repository to. It's
-contained inside a `compatibility` field which specifies the version of Swift
-the commit is known to compile against. Multiple commits compatible with
-different versions of Swift can be specified.
+The `compatibility` field contains a list of version dictionaries, each
+containing a Swift version and a commit. Commits are checked out before
+building a project in the associated Swift version compatibility mode. The
+Swift version is the earliest version of Swift known to compile the project at
+the given commit. The goal is to have multiple commits at different points in a
+project's history that are compatible with all supported Swift version
+compatibility modes.
 
 The `platforms` field specifies the platforms that can be used to build the
 project. Linux and Darwin can currently be specified.
@@ -109,11 +113,12 @@ To add a new Swift Xcode workspace, use the following template:
   "path": "project",
   "branch": "master",
   "maintainer": "[email protected]",
-  "compatibility": {
-    "3.0": {
+  "compatibility": [
+    {
+      "version": "3.0",
       "commit": "195cd8cde2bb717242b3081f9c367ccd0a2f0121"
     }
-  },
+  ],
   "platforms": [
     "Darwin"
   ],
@@ -177,11 +182,12 @@ To add a new Swift Xcode project, use the following template:
   "path": "project",
   "branch": "master",
   "maintainer": "[email protected]",
-  "compatibility": {
-    "3.0": {
+  "compatibility": [
+    {
+      "version": "3.0",
       "commit": "195cd8cde2bb717242b3081f9c367ccd0a2f0121"
     }
-  },
+  ],
   "platforms": [
     "Darwin"
   ],
@@ -206,15 +212,15 @@ in Xcode 8.0.
 # Select Xcode 8.0 GM
 sudo xcode-select -s /Applications/Xcode.app
 # Build project at pinned commit against selected Xcode
-./project_precommit_check project-path-field
+./project_precommit_check project-path-field --earliest-compatible-swift-version 3.0
 ~~~
 
 On Linux, you can build against the Swift 3.0 release toolchain:
 
 ~~~bash
 curl -O https://swift.org/builds/swift-3.0-release/ubuntu1510/swift-3.0-RELEASE/swift-3.0-RELEASE-ubuntu15.10.tar.gz
 tar xzvf swift-3.0-RELEASE-ubuntu15.10.tar.gz
-./project_precommit_check project-path-field --swiftc swift-3.0-RELEASE-ubuntu15.10/usr/bin/swiftc
+./project_precommit_check project-path-field --earliest-compatible-swift-version 3.0 --swiftc swift-3.0-RELEASE-ubuntu15.10/usr/bin/swiftc
 ~~~
 
 ## Maintaining Projects
@@ -238,13 +244,13 @@ To build all projects against a specified Swift compiler locally, use the
 `runner.py` utility as shown below.
 
 ~~~bash
-./runner.py --swift-branch master --projects projects.json --swift-version 3 --include-actions 'action.startswith("Build")' --swiftc path/to/swiftc
+./runner.py --swift-branch master --projects projects.json --include-actions 'action.startswith("Build")' --swiftc path/to/swiftc
 ~~~
 
 Use the `--include-repos` flag to build a specific project.
 
 ~~~bash
-./runner.py --swift-branch master --projects projects.json --swift-version 3 --include-actions 'action.startswith("Build")' --include-repos 'path == "Alamofire"' --swiftc path/to/swiftc
+./runner.py --swift-branch master --projects projects.json --include-actions 'action.startswith("Build")' --include-repos 'path == "Alamofire"' --swiftc path/to/swiftc
 ~~~
 
 By default, build output is redirected to per-action `.log` files in the current

common.py

@@ -410,6 +410,9 @@ def git_update(url, configured_sha, path,
             ))
         else:
             debug_print('current_sha == configured_sha', stderr=stderr)
+            returncodes.append(git_checkout(configured_sha, path,
+                                            force=True,
+                                            stdout=stdout, stderr=stderr))
     except ExecuteCommandFailure:
         debug_print("warning: Unable to update. Falling back to a clone.",
                     stderr=stderr)