LibCore+LibFileSystem: Move resolve_executable_from_environment to Core

Userland/Libraries/LibCore/System.cpp

@@ -1655,4 +1655,39 @@ ErrorOr<void> posix_fallocate(int fd, off_t offset, off_t length)
 }
 #endif
 
+// This constant is copied from LibFileSystem. We cannot use or even include it directly,
+// because that would cause a dependency of LibCore on LibFileSystem, effectively rendering
+// the distinction between these libraries moot.
+static constexpr StringView INTERNAL_DEFAULT_PATH_SV = "/usr/local/sbin:/usr/local/bin:/usr/bin:/bin"sv;
+
+ErrorOr<String> resolve_executable_from_environment(StringView filename, int flags)
+{
+    if (filename.is_empty())
+        return Error::from_errno(ENOENT);
+
+    // Paths that aren't just a file name generally count as already resolved.
+    if (filename.contains('/')) {
+        TRY(Core::System::access(filename, X_OK, flags));
+        return TRY(String::from_utf8(filename));
+    }
+
+    auto const* path_str = ::getenv("PATH");
+    StringView path;
+    if (path_str)
+        path = { path_str, strlen(path_str) };
+    if (path.is_empty())
+        path = INTERNAL_DEFAULT_PATH_SV;
+
+    auto directories = path.split_view(':');
+
+    for (auto directory : directories) {
+        auto file = TRY(String::formatted("{}/{}", directory, filename));
+
+        if (!Core::System::access(file, X_OK, flags).is_error())
+            return file;
+    }
+
+    return Error::from_errno(ENOENT);
+}
+
 }