For corefx testing, use a matching corefx repo version git hash

If the user doesn't specify a specific corefx commit hash to use,
try to find the matching commit hash in the coreclr repro. If the
matching hash can't be found, use 'HEAD'.

We find the matching corefx commit hash by first parsing file
'dependencies.props' at the root of the coreclr repro, looking for this:
   <MicrosoftPrivateCoreFxNETCoreAppPackageVersion>4.5.0-preview1-26112-01</MicrosoftPrivateCoreFxNETCoreAppPackageVersion>
This determines the corefx package version that matches. Next, we look for the version.txt
file in the package cache, e.g.,
   <coreclr_root>\packages\microsoft.private.corefx.netcoreapp\4.5.0-preview1-26112-01\version.txt
The contents of this file is exactly the git commit hash
we need to use, e.g.:
   197a069
The version.txt file is created when the corefx package is restored,
which happens when doing one of:
   Windows: tests\runtests.cmd GenerateLayoutOnly
   non-Windows: build-test.sh generatelayoutonly

It would also be possible to not depend on the package already
being downloaded, but instead download the correct package here,
using the determined "MicrosoftPrivateCoreFxNETCoreAppPackageVersion"
package version, e.g.:
   https://dotnet.myget.org/F/dotnet-core/api/v2/package/Microsoft.Private.CoreFx.NETCoreApp/4.5.0-preview1-26112-01
and then extracting the ZIP archive to find the version.txt file.

This might get easier if the corefx commit hash is added directly to dependencies.props, as
discussed in dotnet/buildtools#1141.

Author: BruceForstall

tests/scripts/run-corefx-tests.py

@@ -120,9 +120,6 @@ def validate_arg(arg, check):
     validate_arg(build_type, lambda item: item in valid_build_types)
     validate_arg(fx_branch, lambda item: True)
 
-    if fx_commit is None:
-        fx_commit = 'HEAD'
-
     if clr_root is None:
         clr_root = nth_dirname(os.path.abspath(sys.argv[0]), 3)
     else:
@@ -198,6 +195,53 @@ def main(args):
                              'Product',
                              '%s.%s.%s' % (clr_os, arch, build_type))
 
+    # If the user doesn't specify a specific corefx commit hash to use, try to find the matching
+    # commit hash in the coreclr repro. If the matching hash can't be found, use 'HEAD'.
+    #
+    # We find the matching corefx commit hash by first parsing file 'dependencies.props' at the root
+    # of the coreclr repro, looking for this:
+    #    <MicrosoftPrivateCoreFxNETCoreAppPackageVersion>4.5.0-preview1-26112-01</MicrosoftPrivateCoreFxNETCoreAppPackageVersion>
+    # This determines the corefx package version that matches. Next, we look for the version.txt
+    # file in the package cache, e.g.,
+    #    <coreclr_root>\packages\microsoft.private.corefx.netcoreapp\4.5.0-preview1-26112-01\version.txt
+    # The contents of this file is exactly the git commit hash we need to use, e.g.:
+    #    197a0699b08087ea85581679afdd9fd7b5c465c3
+    # The version.txt file is created when the corefx package is restored, which happens when doing one of:
+    #    Windows: tests\runtests.cmd GenerateLayoutOnly
+    #    non-Windows: build-test.sh generatelayoutonly
+    #
+    # It would also be possible to not depend on the package already being downloaded, but instead
+    # download the correct package here, using the determined "MicrosoftPrivateCoreFxNETCoreAppPackageVersion"
+    # package version, e.g.:
+    #    https://dotnet.myget.org/F/dotnet-core/api/v2/package/Microsoft.Private.CoreFx.NETCoreApp/4.5.0-preview1-26112-01
+    # and then extracting the ZIP archive to find the version.txt file.
+    #
+    # This might get easier if the corefx commit hash is added directly to dependencies.props, as
+    # discussed in https://github.com/dotnet/buildtools/issues/1141.
+
+    if fx_commit is None:
+        # Default to 'HEAD'; overwrite if we find an actual commit hash.
+        fx_commit = 'HEAD'
+        try:
+            dependencies_filename = os.path.join(clr_root, 'dependencies.props')
+            if os.path.isfile(dependencies_filename):
+                with open(dependencies_filename, 'r') as dependencies_file_handle:
+                    dependencies_file = dependencies_file_handle.read()
+                matchObj = re.search(r'.*<MicrosoftPrivateCoreFxNETCoreAppPackageVersion>(.+)</MicrosoftPrivateCoreFxNETCoreAppPackageVersion>.*', dependencies_file)
+                if matchObj:
+                    package_version_string = matchObj.group(1)
+                    version_filename = os.path.join(clr_root, 'packages', 'microsoft.private.corefx.netcoreapp', package_version_string, 'version.txt')
+                    if os.path.isfile(version_filename):
+                        with open(version_filename, 'r') as f:
+                            version_file = f.readlines()
+                        fx_commit = version_file[0].strip()
+                        log("Using matching corefx commit hash: %s" % fx_commit)
+        except:
+            log("Failed to find matching corefx commit hash")
+
+        if fx_commit == 'HEAD':
+            log("Using default corefx commit hash: HEAD")
+
     # corefx creates both files that are read-only and files that include non-ascii
     # characters. Using onerror=del_rw allows us to delete all of the read-only files.
     # To delete the files with non-ascii characters, when rmtree fails due to those
@@ -250,9 +294,6 @@ def main(args):
         os.putenv('HOME', fx_home)
         log('HOME=' + fx_home)
 
-    # Determine the RID to specify the to corefix build scripts.  This seems to
-    # be way harder than it ought to be.
- 
     # Gather up some arguments to pass to both build and build-tests.
 
     config_args = '-Release -os:%s -buildArch:%s' % (clr_os, arch)