locator: correctly reject patches adding a file for non-empty files

include/patch/locator.h

@@ -29,6 +29,8 @@ struct Location {
     LineNumber offset { -1 };
 };
 
+LineNumber expected_line_number(const Hunk& hunk);
+
 Location locate_hunk(const std::vector<Line>& content, const Hunk& hunk, bool ignore_whitespace = false, LineNumber offset = 0, LineNumber max_fuzz = 2);
 
 bool matches_ignoring_whitespace(const std::string& as, const std::string& bs);

src/applier.cpp

@@ -167,7 +167,7 @@ static void print_hunk_statistics(std::ostream& out, size_t hunk_num, bool skipp
         }
         out << ".\n";
     } else {
-        out << hunk.old_file_range.start_line + offset_old_lines_to_new << ".\n";
+        out << expected_line_number(hunk) + offset_old_lines_to_new << ".\n";
     }
 }
 

src/locator.cpp

@@ -80,7 +80,7 @@ bool matches(const Line& line1, const Line& line2, bool ignore_whitespace)
     return matches_ignoring_whitespace(line1.content, line2.content);
 }
 
-static LineNumber expected_line_number(const Hunk& hunk)
+LineNumber expected_line_number(const Hunk& hunk)
 {
     auto line = hunk.old_file_range.start_line;
     if (hunk.old_file_range.number_of_lines == 0)
@@ -93,6 +93,21 @@ Location locate_hunk(const std::vector<Line>& content, const Hunk& hunk, bool ig
     // Make a first best guess at where the from-file range is telling us where the hunk should be.
     LineNumber offset_guess = expected_line_number(hunk) - 1 + offset;
 
+    // If there's no lines surrounding this hunk - it will always succeed,
+    // so there is no point in checking any further. Note that this check is
+    // also what makes matching against an empty 'from file' work (with no lines),
+    // as in that case there is no content for us to even match against in the
+    // first place!
+    //
+    // Furthermore, we also should reject patches being added when the hunk is
+    // claiming the file is completely empty - but there are actually lines in
+    // that file.
+    if (hunk.old_file_range.number_of_lines == 0) {
+        if (hunk.old_file_range.start_line == 0 && !content.empty())
+            return {};
+        return { offset_guess, 0, 0 };
+    }
+
     LineNumber patch_prefix_content = 0;
     for (const auto& line : hunk.lines) {
         if (line.operation != ' ')
@@ -109,14 +124,6 @@ Location locate_hunk(const std::vector<Line>& content, const Hunk& hunk, bool ig
 
     LineNumber context = std::max(patch_prefix_content, patch_suffix_content);
 
-    // If there's no lines surrounding this hunk - it will always succeed,
-    // so there is no point in checking any further. Note that this check is
-    // also what makes matching against an empty 'from file' work (with no lines),
-    // as in that case there is no content for us to even match against in the
-    // first place!
-    if (hunk.old_file_range.number_of_lines == 0)
-        return { offset_guess, 0, 0 };
-
     for (LineNumber fuzz = 0; fuzz <= max_fuzz; ++fuzz) {
 
         auto suffix_fuzz = std::max<LineNumber>(fuzz + patch_suffix_content - context, 0);