log: Fall back to CWD-relative .nvimlog

If if the resolved $NVIM_LOG_FILE *and* stdpath("data")/log cannot be
created (e.g. because the XDG data directory does not exist), fall back
to .nvimlog in the current direcrtory.

src/nvim/log.c

@@ -25,6 +25,19 @@ static uv_mutex_t mutex;
 # include "log.c.generated.h"
 #endif
 
+static bool log_try_create(char *fname)
+{
+  if (fname == NULL || fname[0] == '\0') {
+    return false;
+  }
+  FILE *log_file = fopen(fname, "a");
+  if (log_file == NULL) {
+    return false;
+  }
+  fclose(log_file);
+  return true;
+}
+
 /// Initializes path to log file. Sets $NVIM_LOG_FILE if empty.
 ///
 /// Tries $NVIM_LOG_FILE, or falls back to $XDG_DATA_HOME/nvim/log. Path to log
@@ -43,17 +56,22 @@ static bool log_path_init(void)
              (int)size - 1);
   if (strequal("$" LOG_FILE_ENV, log_file_path)
       || log_file_path[0] == '\0'
-      || os_isdir((char_u *)log_file_path)) {
+      || os_isdir((char_u *)log_file_path)
+      || !log_try_create(log_file_path)) {
     // Invalid $NVIM_LOG_FILE or failed to expand; fall back to default.
-    memset(log_file_path, 0, size);
     char *defaultpath = stdpaths_user_data_subpath("log", 0, true);
     size_t len = xstrlcpy(log_file_path, defaultpath, size);
-    if (len >= size) {  // Fall back to stderr.
-      memset(log_file_path, 0, size);
+    xfree(defaultpath);
+    // Fall back to .nvimlog
+    if (len >= size || !log_try_create(log_file_path)) {
+      len = xstrlcpy(log_file_path, ".nvimlog", size);
+    }
+    // Fall back to stderr
+    if (len >= size || !log_try_create(log_file_path)) {
+      log_file_path[0] = '\0';
       return false;
     }
     os_setenv(LOG_FILE_ENV, log_file_path, true);
-    xfree(defaultpath);
   }
   return true;
 }

test/functional/options/defaults_spec.lua

@@ -8,6 +8,8 @@ local clear = helpers.clear
 local eval = helpers.eval
 local eq = helpers.eq
 local neq = helpers.neq
+local mkdir = helpers.mkdir
+local rmdir = helpers.rmdir
 
 local function init_session(...)
   local args = { helpers.nvim_prog, '-i', 'NONE', '--embed',
@@ -123,32 +125,52 @@ describe('startup defaults', function()
   end)
 
   describe('$NVIM_LOG_FILE', function()
+    -- TODO(jkeyes): use stdpath('data') instead.
+    local datasubdir = helpers.iswin() and 'nvim-data' or 'nvim'
+    local xdgdir = 'Xtest-startup-xdg-logpath'
+    local xdgdatadir = xdgdir..'/'..datasubdir
     after_each(function()
       os.remove('Xtest-logpath')
+      rmdir(xdgdir)
     end)
+
     it('is used if expansion succeeds', function()
       clear({env={
         NVIM_LOG_FILE='Xtest-logpath',
       }})
       eq('Xtest-logpath', eval('$NVIM_LOG_FILE'))
     end)
-    it('defaults to stdpath("data")/log', function()
+    it('defaults to stdpath("data")/log if empty', function()
+      eq(true, mkdir(xdgdir) and mkdir(xdgdatadir))
       clear({env={
-        XDG_DATA_HOME='Xtest-startup-logpath',
-        NVIM_LOG_FILE='',  -- Empty value is considered invalid.
+        XDG_DATA_HOME=xdgdir,
+        NVIM_LOG_FILE='',  -- Empty is invalid.
       }})
-      -- TODO(jkeyes): use stdpath('data') instead.
-      local dir = helpers.iswin() and 'nvim-data' or 'nvim'
-      eq('Xtest-startup-logpath/'..dir..'/log', string.gsub(eval('$NVIM_LOG_FILE'), '\\', '/'))
+      -- server_start() calls ELOG, which tickles log_path_init().
+      pcall(command, 'call serverstart(serverlist()[0])')
+
+      eq(xdgdir..'/'..datasubdir..'/log', string.gsub(eval('$NVIM_LOG_FILE'), '\\', '/'))
     end)
-    it('if invalid, falls back to default', function()
+    it('defaults to stdpath("data")/log if invalid', function()
+      eq(true, mkdir(xdgdir) and mkdir(xdgdatadir))
       clear({env={
-        XDG_DATA_HOME='Xtest-startup-logpath',
-        NVIM_LOG_FILE='.',  -- Directory is considered invalid.
+        XDG_DATA_HOME=xdgdir,
+        NVIM_LOG_FILE='.',  -- Any directory is invalid.
       }})
-      -- TODO(jkeyes): use stdpath('data') instead.
-      local dir = helpers.iswin() and 'nvim-data' or 'nvim'
-      eq('Xtest-startup-logpath/'..dir..'/log', string.gsub(eval('$NVIM_LOG_FILE'), '\\', '/'))
+      -- server_start() calls ELOG, which tickles log_path_init().
+      pcall(command, 'call serverstart(serverlist()[0])')
+
+      eq(xdgdir..'/'..datasubdir..'/log', string.gsub(eval('$NVIM_LOG_FILE'), '\\', '/'))
+    end)
+    it('defaults to .nvimlog if stdpath("data") is invalid', function()
+      clear({env={
+        XDG_DATA_HOME='Xtest-missing-xdg-dir',
+        NVIM_LOG_FILE='.',  -- Any directory is invalid.
+      }})
+      -- server_start() calls ELOG, which tickles log_path_init().
+      pcall(command, 'call serverstart(serverlist()[0])')
+
+      eq('.nvimlog', eval('$NVIM_LOG_FILE'))
     end)
   end)
 end)