On *nix, use a realpath(3) variant (which does not expand symlinks) in NormalizePath

Before this commit, when a source file is a symlink, cquery is not able to serve LSP requests for it.

src/platform_linux.cc

@@ -29,10 +29,8 @@
 #include <fcntl.h>
 #include <unistd.h>
 
-#include <fcntl.h> /* For O_* constants */
 #include <semaphore.h>
 #include <sys/mman.h>
-#include <sys/stat.h> /* For mode constants */
 
 #ifndef __APPLE__
 #include <sys/prctl.h>
@@ -42,6 +40,73 @@
 #include <malloc.h>
 #endif
 
+namespace {
+
+// Returns the canonicalized absolute pathname, without expanding symbolic links.
+// This is a variant of realpath(2), C++ rewrite of
+// https://github.com/freebsd/freebsd/blob/master/lib/libc/stdlib/realpath.c
+optional<std::string> RealPathNotExpandSymlink(std::string path) {
+  if (path.empty()) {
+    errno = EINVAL;
+    return nullopt;
+  }
+  if (path[0] == '\0') {
+    errno = ENOENT;
+    return nullopt;
+  }
+
+  // Do not use PATH_MAX because it is tricky on Linux.
+  // See https://eklitzke.org/path-max-is-tricky
+  char tmp[1024];
+  std::string resolved;
+  size_t i = 0;
+  struct stat sb;
+  if (path[0] == '/') {
+    resolved = "/";
+    i = 1;
+  } else {
+    if (!getcwd(tmp, sizeof tmp))
+      return nullopt;
+    resolved = tmp;
+  }
+
+  while (i < path.size()) {
+    auto j = path.find('/', i);
+    if (j == std::string::npos)
+      j = path.size();
+    auto next_token = path.substr(i, j - i);
+    i = j + 1;
+    if (resolved.back() != '/')
+      resolved += '/';
+    if (next_token.empty() || next_token == ".") {
+      // Handle consequential slashes and "."
+      continue;
+    } else if (next_token == "..") {
+      // Strip the last path component except when it is single "/"
+      if (resolved.size() > 1)
+        resolved.resize(resolved.rfind('/', resolved.size() - 2) + 1);
+      continue;
+    }
+    // Append the next path component.
+    // Here we differ from realpath(3), we use stat(2) instead of
+    // lstat(2) because we do not want to resolve symlinks.
+    resolved += next_token;
+    if (stat(resolved.c_str(), &sb) != 0)
+      return nullopt;
+    if (!S_ISDIR(sb.st_mode) && j < path.size()) {
+      errno = ENOTDIR;
+      return nullopt;
+    }
+  }
+
+  // Remove trailing slash except when a single "/".
+  if (resolved.size() > 1 && resolved.back() == '/')
+    resolved.pop_back();
+  return resolved;
+}
+
+}  // namespace
+
 struct PlatformMutexLinux : public PlatformMutex {
   sem_t* sem_ = nullptr;
 
@@ -155,12 +220,8 @@ std::string GetWorkingDirectory() {
 }
 
 std::string NormalizePath(const std::string& path) {
-  errno = 0;
-  char name[PATH_MAX + 1];
-  (void)realpath(path.c_str(), name);
-  if (errno)
-    return path;
-  return name;
+  optional<std::string> resolved = RealPathNotExpandSymlink(path);
+  return resolved ? *resolved : path;
 }
 
 bool TryMakeDirectory(const std::string& absolute_path) {