IDEA-157915 resolve Git symbolic-ref in HEAD

plugins/git4idea/src/git4idea/repo/GitRepositoryFiles.java

@@ -316,4 +316,9 @@ public class GitRepositoryFiles {
   Collection<VirtualFile> getRootDirs() {
     return ContainerUtil.newHashSet(myMainDir, myWorktreeDir);
   }
+
+  @NotNull
+  File getBranchFile(@NotNull String fullBranchName) {
+    return file(myMainDir.getPath() + slash(fullBranchName));
+  }
 }

plugins/git4idea/src/git4idea/repo/GitRepositoryReader.java

@@ -23,6 +23,7 @@ import com.intellij.openapi.util.Condition;
 import com.intellij.openapi.util.Pair;
 import com.intellij.openapi.util.io.FileUtil;
 import com.intellij.openapi.util.text.LineTokenizer;
+import com.intellij.openapi.util.text.StringUtil;
 import com.intellij.openapi.vfs.CharsetToolkit;
 import com.intellij.util.Function;
 import com.intellij.util.Processor;
@@ -320,9 +321,14 @@ class GitRepositoryReader {
 
   @NotNull
   private HeadInfo readHead() {
+    return readHeadInternal(myHeadFile, new ArrayList<>());
+  }
+
+  @NotNull
+  private HeadInfo readHeadInternal(@NotNull File headFile, @NotNull List<String> alreadyVisited) {
     String headContent;
     try {
-      headContent = DvcsUtil.tryLoadFile(myHeadFile, CharsetToolkit.UTF8);
+      headContent = DvcsUtil.tryLoadFile(headFile, CharsetToolkit.UTF8);
     }
     catch (RepoStateException e) {
       LOG.error(e);
@@ -331,11 +337,22 @@ class GitRepositoryReader {
 
     Hash hash = parseHash(headContent);
     if (hash != null) {
-      return new HeadInfo(false, headContent);
+      if (alreadyVisited.isEmpty()) {
+        return new HeadInfo(false, headContent);
+      } else {
+        return new HeadInfo(true, ContainerUtil.getLastItem(alreadyVisited));
+      }
     }
     String target = getTarget(headContent);
     if (target != null) {
-      return new HeadInfo(true, target);
+      if (alreadyVisited.contains(target)) {
+        alreadyVisited.add(target);
+        LOG.error(new RepoStateException("Cyclic symbolic ref in HEAD: [" + StringUtil.join(alreadyVisited, " -> ") + "]"));
+        return new HeadInfo(false, null);
+      } else {
+        alreadyVisited.add(target);
+        return readHeadInternal(myGitFiles.getBranchFile(target), alreadyVisited);
+      }
     }
     LOG.error(new RepoStateException("Invalid format of the .git/HEAD file: [" + headContent + "]")); // including "refs/tags/v1"
     return new HeadInfo(false, null);

plugins/git4idea/testData/repo/symbolic-refs/current-branch.txt

@@ -1 +1 @@
-0e1d130689bc52f140c5c374aa9cc2b8916c0ad7 master-link
+0e1d130689bc52f140c5c374aa9cc2b8916c0ad7 master