GitInput: Determine default branch for repository

Up until now when no branch was defined the Git Plugin hardcoded the
master branch.

Now we try resolving the default branch with git ls-remote and allow
a configurable fallback branch, that can be master or anything else.

Author: mweinelt

subprojects/hydra/lib/Hydra/Plugin/GitInput.pm

@@ -28,7 +28,6 @@ sub _parseValue {
     my ($value) = @_;
     my @parts = split ' ', $value;
     (my $uri, my $branch, my $deepClone) = @parts;
-    $branch = defined $branch ? $branch : "master";
     my $options = {};
     my $start_options = 3;
     # if deepClone has "=" then is considered an option
@@ -44,6 +43,24 @@ sub _parseValue {
     return ($uri, $branch, $deepClone, $options);
 }
 
+sub _getDefaultBranch {
+    my ($uri) = @_;
+
+    my $res = runCommand(
+        cmd => ["git", "ls-remote", "--symref", $uri, "HEAD"],
+    );
+
+    return undef if $res->{status};
+
+    for my $line (split /\n/, ($res->{stdout} // '')) {
+        if ($line =~ /^ref:\s+refs\/heads\/(\S+)/) {
+            return $1;
+        }
+    }
+
+    return undef;
+}
+
 sub _printIfDebug {
     my ($msg) = @_;
     print STDERR "GitInput: $msg" if $ENV{'HYDRA_DEBUG'};
@@ -55,7 +72,8 @@ Read the configuration from the main hydra config file.
 
 The configuration is loaded from the "git-input" block.
 
-Currently only the "timeout" variable is been looked up in the file.
+Currently the "fallback_branch" and a "timeout" variable are looked up in the
+file.
 
 The variables defined directly in the input value will override
 the ones on the configuration file, to define the variables
@@ -67,6 +85,9 @@ Expected configuration format in the hydra config file:
       # general timeout
       timeout = 400
 
+      # branch, if no default branch can be determined and none is specified
+      fallback_branch = main
+
       <project:jobset:input-name>
         # specific timeout for a particular input
         timeout = 400
@@ -79,6 +100,7 @@ sub _pluginConfig {
     my $cfg = $main_config->{$CONFIG_SECTION};
     # default values
     my $values = {
+        fallback_branch => "master",
         timeout => 600,
     };
     my $input_block = "$project_name:$jobset_name:$input_name";
@@ -126,6 +148,11 @@ sub fetchInput {
         _printIfDebug "'$name': override '$opt_name' with input value: $opt_value\n";
     }
 
+    # Determine a branch to check out, if none is explicitly defined
+    if (!defined $branch) {
+        $branch = _getDefaultBranch($uri) // $cfg->{fallback_branch} // "master";
+    }
+
     # Clone or update a branch of the repository into our SCM cache.
     my $cacheDir = getSCMCacheDir . "/git";
     mkpath($cacheDir);