fix: CWD restore, install error checks, fetch EINTR

- build.cpp: save/restore CWD on both success and failure paths
- install.cpp: check error_code after each filesystem op, abort on failure
- install.cpp: check write_installed() return value
- fetch.cpp: EINTR retry loop on both waitpid calls (exec_cmd + exec_capture)

src/install/install.cpp

@@ -42,21 +42,26 @@ InstallResult install(const resolve::BuildStep& step,
     auto src = work_dir / "destdir";
     if (fs::exists(src)) {
         for (auto& entry : fs::recursive_directory_iterator(src, ec)) {
+            if (ec) { break; }
             auto rel = fs::relative(entry.path(), src);
             auto target = dest / rel;
             if (entry.is_directory()) {
                 fs::create_directories(target, ec);
             } else {
                 fs::create_directories(target.parent_path(), ec);
-                fs::rename(entry.path(), target, ec);
+                if (!ec) { fs::rename(entry.path(), target, ec); }
             }
+            if (ec) { break; }
         }
     }
 
     auto entries = read_installed();
     entries.push_back({step.name, step.resolved.original.version,
                        hash, step.resolved.original.provides});
-    write_installed(entries);
+    if (!write_installed(entries)) {
+        result.error = "failed to write installed DB";
+        return result;
+    }
 
     std::vector<std::string> hashes;
     for (auto& e : entries) { hashes.push_back(e.hash); }