From cfa9c83d33ab24c39df3bd74459176354849c4e1 Mon Sep 17 00:00:00 2001
From: asrelo <47759736+asrelo@users.noreply.github.com>
Date: Sun, 11 Aug 2024 21:19:14 +0300
Subject: [PATCH] Improve fs::PathStartsWith to handle empty strings (#14877)

 `""` does not refer to a proper path, and `fs::PathStartsWith(path, "")` should just return `false`. This is also the case in libraries in other languages where I looked, seems to be common.

The new behavior:
* check early, if `prefix` is empty - return if path is empty or not,
* no special processing for when `path` is empty, the function meets characters in `prefix` and returns false anyway.
---
 src/filesys.cpp               | 11 +++++++++++
 src/unittest/test_filesys.cpp | 23 ++++++++++++-----------
 2 files changed, 23 insertions(+), 11 deletions(-)

diff --git a/src/filesys.cpp b/src/filesys.cpp
index 733a3b203..4287c8b05 100644
--- a/src/filesys.cpp
+++ b/src/filesys.cpp
@@ -697,32 +697,43 @@ bool MoveDir(const std::string &source, const std::string &target)
 
 bool PathStartsWith(const std::string &path, const std::string &prefix)
 {
+	if (prefix.empty())
+		return path.empty();
 	size_t pathsize = path.size();
 	size_t pathpos = 0;
 	size_t prefixsize = prefix.size();
 	size_t prefixpos = 0;
 	for(;;){
+		// Test if current characters at path and prefix are delimiter OR EOS
 		bool delim1 = pathpos == pathsize
 			|| IsDirDelimiter(path[pathpos]);
 		bool delim2 = prefixpos == prefixsize
 			|| IsDirDelimiter(prefix[prefixpos]);
 
+		// Return false if it's delimiter/EOS in one path but not in the other
 		if(delim1 != delim2)
 			return false;
 
 		if(delim1){
+			// Skip consequent delimiters in path, in prefix
 			while(pathpos < pathsize &&
 					IsDirDelimiter(path[pathpos]))
 				++pathpos;
 			while(prefixpos < prefixsize &&
 					IsDirDelimiter(prefix[prefixpos]))
 				++prefixpos;
+			// Return true if prefix has ended (at delimiter/EOS)
 			if(prefixpos == prefixsize)
 				return true;
+			// Return false if path has ended (at delimiter/EOS)
+            // while prefix did not.
 			if(pathpos == pathsize)
 				return false;
 		}
 		else{
+			// Skip pairwise-equal characters in path and prefix until
+			// delimiter/EOS in path or prefix.
+			// Return false if differing characters are met.
 			size_t len = 0;
 			do{
 				char pathchar = path[pathpos+len];
diff --git a/src/unittest/test_filesys.cpp b/src/unittest/test_filesys.cpp
index 8fc9db3d8..fd25d2de9 100644
--- a/src/unittest/test_filesys.cpp
+++ b/src/unittest/test_filesys.cpp
@@ -113,6 +113,7 @@ void TestFileSys::testPathStartsWith()
 	};
 	/*
 		expected fs::PathStartsWith results
+		(row for every path, column for every prefix)
 		0 = returns false
 		1 = returns true
 		2 = returns false on windows, true elsewhere
@@ -122,17 +123,17 @@ void TestFileSys::testPathStartsWith()
 	*/
 	int expected_results[numpaths][numpaths] = {
 		{1,2,0,0,0,0,0,0,0,0,0,0},
-		{1,1,0,0,0,0,0,0,0,0,0,0},
-		{1,1,1,0,0,0,0,0,0,0,0,0},
-		{1,1,1,1,0,0,0,0,0,0,0,0},
-		{1,1,0,0,1,0,0,0,0,0,0,0},
-		{1,1,0,0,0,1,0,0,1,1,0,0},
-		{1,1,0,0,0,0,1,4,1,0,0,0},
-		{1,1,0,0,0,0,4,1,4,0,0,0},
-		{1,1,0,0,0,0,0,0,1,0,0,0},
-		{1,1,0,0,0,0,0,0,1,1,0,0},
-		{1,1,0,0,0,0,0,0,0,0,1,0},
-		{1,1,0,0,0,0,0,0,0,0,0,1},
+		{0,1,0,0,0,0,0,0,0,0,0,0},
+		{0,1,1,0,0,0,0,0,0,0,0,0},
+		{0,1,1,1,0,0,0,0,0,0,0,0},
+		{0,1,0,0,1,0,0,0,0,0,0,0},
+		{0,1,0,0,0,1,0,0,1,1,0,0},
+		{0,1,0,0,0,0,1,4,1,0,0,0},
+		{0,1,0,0,0,0,4,1,4,0,0,0},
+		{0,1,0,0,0,0,0,0,1,0,0,0},
+		{0,1,0,0,0,0,0,0,1,1,0,0},
+		{0,1,0,0,0,0,0,0,0,0,1,0},
+		{0,1,0,0,0,0,0,0,0,0,0,1},
 	};
 
 	for (int i = 0; i < numpaths; i++)