Introduce a writemethod option to either overwrite or replace files

This permit to choose if files should be written by overwriting their
content (the default), or by writing to a separate temporary file
and rename it to the current file.

As discussed in #2036

4 files changed, 63 insertions, 24 deletions

diff --git a/src/commands.cc b/src/commands.cc
index 6cc48dbf..3bc145bc 100644
--- a/src/commands.cc
+++ b/src/commands.cc
@@ -342,9 +342,10 @@ void do_write_buffer(Context& context, Optional<String> filename, WriteFlags fla
         throw runtime_error("cannot overwrite the buffer when in readonly mode");
 
     auto effective_filename = not filename ? buffer.name() : parse_filename(*filename);
+    auto mode = context.options()["writemethod"].get<WriteMethod>();
 
     context.hooks().run_hook(Hook::BufWritePre, effective_filename, context);
-    write_buffer_to_file(buffer, effective_filename, flags);
+    write_buffer_to_file(buffer, effective_filename, mode, flags);
     context.hooks().run_hook(Hook::BufWritePost, effective_filename, context);
 }
 
@@ -395,8 +396,9 @@ void write_all_buffers(Context& context, bool sync = false)
              buffer->is_modified())
             and !(buffer->flags() & Buffer::Flags::ReadOnly))
         {
+            auto mode = context.options()["writemethod"].get<WriteMethod>();
             buffer->run_hook_in_own_context(Hook::BufWritePre, buffer->name(), context.name());
-            write_buffer_to_file(*buffer, buffer->name(), sync ? WriteFlags::Sync : WriteFlags::None);
+            write_buffer_to_file(*buffer, buffer->name(), mode, sync ? WriteFlags::Sync : WriteFlags::None);
             buffer->run_hook_in_own_context(Hook::BufWritePost, buffer->name(), context.name());
         }
     }
diff --git a/src/file.cc b/src/file.cc
index d5e73bed..eb26ddc4 100644
--- a/src/file.cc
+++ b/src/file.cc
@@ -280,25 +280,51 @@ void write_buffer_to_fd(Buffer& buffer, int fd)
     }
 }
 
-void write_buffer_to_file(Buffer& buffer, StringView filename, WriteFlags flags)
+int open_temp_file(StringView filename, char (&buffer)[PATH_MAX])
+{
+    String path = real_path(filename);
+    auto [dir,file] = split_path(path);
+
+    if (dir.empty())
+        format_to(buffer, ".{}.kak.XXXXXX", file);
+    else
+        format_to(buffer, "{}/.{}.kak.XXXXXX", dir, file);
+
+    return mkstemp(buffer);
+}
+
+int open_temp_file(StringView filename)
+{
+    char buffer[PATH_MAX];
+    return open_temp_file(filename, buffer);
+}
+
+void write_buffer_to_file(Buffer& buffer, StringView filename,
+                          WriteMethod method, WriteFlags flags)
 {
-    struct stat st;
     auto zfilename = filename.zstr();
+    struct stat st;
+
+    bool replace = method == WriteMethod::Replace;
+    bool force = flags & WriteFlags::Force;
 
-    if (flags & WriteFlags::Force and ::stat(zfilename, &st) == 0)
+    if ((replace or force) and ::stat(zfilename, &st) != 0)
     {
-        if (::chmod(zfilename, st.st_mode | S_IWUSR) < 0)
-            throw runtime_error("unable to change file permissions");
+        force = false;
+        replace = false;
     }
-    else
-        flags |= ~WriteFlags::Force;
+
+    if (force and ::chmod(zfilename, st.st_mode | S_IWUSR) < 0)
+        throw runtime_error("unable to change file permissions");
 
     auto restore_mode = on_scope_end([&]{
-        if (flags & WriteFlags::Force and ::chmod(zfilename, st.st_mode) < 0)
+        if ((force or replace) and ::chmod(zfilename, st.st_mode) < 0)
             throw runtime_error("unable to restore file permissions");
     });
 
-    const int fd = open(zfilename, O_CREAT | O_WRONLY | O_TRUNC, 0644);
+    char temp_filename[PATH_MAX];
+    const int fd = replace ? open_temp_file(filename, temp_filename)
+                           : open(zfilename, O_CREAT | O_WRONLY | O_TRUNC, 0644);
     if (fd == -1)
         throw file_access_error(filename, strerror(errno));
 
@@ -309,6 +335,9 @@ void write_buffer_to_file(Buffer& buffer, StringView filename, WriteFlags flags)
             ::fsync(fd);
     }
 
+    if (replace)
+        rename(temp_filename, zfilename);
+
     if ((buffer.flags() & Buffer::Flags::File) and
         real_path(filename) == real_path(buffer.name()))
         buffer.notify_saved();
@@ -316,16 +345,7 @@ void write_buffer_to_file(Buffer& buffer, StringView filename, WriteFlags flags)
 
 void write_buffer_to_backup_file(Buffer& buffer)
 {
-    String path = real_path(buffer.name());
-    auto [dir,file] = split_path(path);
-
-    char pattern[PATH_MAX];
-    if (dir.empty())
-        format_to(pattern, ".{}.kak.XXXXXX", file);
-    else
-        format_to(pattern, "{}/.{}.kak.XXXXXX", dir, file);
-
-    int fd = mkstemp(pattern);
+    const int fd = open_temp_file(buffer.name());
     if (fd >= 0)
     {
         write_buffer_to_fd(buffer, fd);
diff --git a/src/file.hh b/src/file.hh
index db4ac6ff..6871a1a3 100644
--- a/src/file.hh
+++ b/src/file.hh
@@ -2,6 +2,7 @@
 #define file_hh_INCLUDED
 
 #include "array_view.hh"
+#include "enum.hh"
 #include "meta.hh"
 #include "string.hh"
 #include "units.hh"
@@ -51,6 +52,19 @@ struct MappedFile
     struct stat st {};
 };
 
+enum class WriteMethod
+{
+    Overwrite,
+    Replace
+};
+constexpr auto enum_desc(Meta::Type<WriteMethod>)
+{
+    return make_array<EnumDesc<WriteMethod>, 2>({
+        { WriteMethod::Overwrite, "overwrite" },
+        { WriteMethod::Replace, "replace" },
+    });
+}
+
 enum class WriteFlags
 {
     None  = 0,
@@ -60,7 +74,7 @@ enum class WriteFlags
 constexpr bool with_bit_ops(Meta::Type<WriteFlags>) { return true; }
 
 void write_buffer_to_file(Buffer& buffer, StringView filename,
-                          WriteFlags flags);
+                          WriteMethod method, WriteFlags flags);
 void write_buffer_to_fd(Buffer& buffer, int fd);
 void write_buffer_to_backup_file(Buffer& buffer);
 
diff --git a/src/main.cc b/src/main.cc
index 95d5ca41..3cd9bc50 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -399,6 +399,9 @@ void register_options()
     reg.declare_option("autoreload",
                        "autoreload buffer when a filesystem modification is detected",
                        Autoreload::Ask);
+    reg.declare_option("writemethod",
+                       "how to write buffer to files",
+                       WriteMethod::Overwrite);
     reg.declare_option<int, check_timeout>(
         "idle_timeout", "timeout, in milliseconds, before idle hooks are triggered", 50);
     reg.declare_option<int, check_timeout>(
@@ -848,10 +851,10 @@ int run_filter(StringView keystr, ConstArrayView<StringView> files, bool quiet,
             Buffer* buffer = open_file_buffer(file, Buffer::Flags::NoHooks);
             if (not suffix_backup.empty())
                 write_buffer_to_file(*buffer, buffer->name() + suffix_backup,
-                                     WriteFlags::None);
+                                     WriteMethod::Overwrite, WriteFlags::None);
             apply_to_buffer(*buffer);
             write_buffer_to_file(*buffer, buffer->name(),
-                                 WriteFlags::None);
+                                 WriteMethod::Overwrite, WriteFlags::None);
             buffer_manager.delete_buffer(*buffer);
         }
         if (not isatty(0))