vim-patch:9.2.0061: Not possible to know when a session will be loaded (#38071)

Problem:  Not possible to know when a session will be loaded.
Solution: Add the SessionLoadPre autocommand (Colin Kennedy).

fixes:  vim/vim#19084
closes: vim/vim#19306

1c0d468d72

Co-authored-by: Colin Kennedy <colinvfx@gmail.com>

runtime/doc/autocmd.txt

@@ -961,6 +961,9 @@ SafeState			When nothing is pending, going to wait for the
 				check more with `state()`, e.g. whether the
 				screen was scrolled for messages.
 
+							*SessionLoadPre*
+SessionLoadPre			Before loading the session file created using
+				the |:mksession| command.
 							*SessionLoadPost*
 SessionLoadPost			After loading the session file created using
 				the |:mksession| command.

runtime/doc/news.txt

@@ -240,6 +240,7 @@ EVENTS
 • Creating or updating a progress message with |nvim_echo()| triggers a |Progress| event.
 • |MarkSet| is triggered after a |mark| is set by the user (currently doesn't
   support implicit marks like |'[| or |'<|, …).
+• |SessionLoadPre| is triggered before loading a |Session| file.
 • |TabClosedPre| is triggered before closing a |tabpage|.
 • New `terminator` parameter for |TermRequest| event.
 

runtime/doc/options.txt

@@ -2553,6 +2553,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 		|RemoteReply|,
 		|SafeState|,
 		|SessionLoadPost|,
+		|SessionLoadPre|,
 		|SessionWritePost|,
 		|ShellCmdPost|,
 		|Signal|,

runtime/doc/starting.txt

@@ -852,8 +852,8 @@ This saves the current Session, and starts off the command to load another.
 A session includes all tab pages, unless "tabpages" was removed from
 'sessionoptions'. |tab-page|
 
-The |SessionLoadPost| autocmd event is triggered after a session file is
-loaded/sourced.
+The |SessionLoadPre| autocmd event is triggered before a session file is
+loaded/sourced and |SessionLoadPost| autocmd event is triggered after.
 						*SessionLoad-variable*
 While the session file is loading, the SessionLoad global variable is set to
 1.  Plugins can use this to postpone some work until the SessionLoadPost event

runtime/lua/vim/_meta/api_keysets.lua

@@ -176,6 +176,7 @@ error('Cannot require a meta file')
 --- |'SafeState'
 --- |'SearchWrapped'
 --- |'SessionLoadPost'
+--- |'SessionLoadPre'
 --- |'SessionWritePost'
 --- |'ShellCmdPost'
 --- |'ShellFilterPost'

runtime/lua/vim/_meta/options.lua

@@ -2231,6 +2231,7 @@ vim.go.ei = vim.go.eventignore
 --- 	`RemoteReply`,
 --- 	`SafeState`,
 --- 	`SessionLoadPost`,
+--- 	`SessionLoadPre`,
 --- 	`SessionWritePost`,
 --- 	`ShellCmdPost`,
 --- 	`Signal`,

src/nvim/auevents.lua

@@ -97,6 +97,7 @@ return {
     SafeState = false, -- going to wait for a character
     SearchWrapped = true, -- after the search wrapped around
     SessionLoadPost = false, -- after loading a session file
+    SessionLoadPre = false, -- before loading a session file
     SessionWritePost = false, -- after writing a session file
     ShellCmdPost = false, -- after ":!cmd"
     ShellFilterPost = true, -- after ":1,2!cmd", ":w !cmd", ":r !cmd".

src/nvim/ex_session.c

@@ -600,6 +600,9 @@ static int makeopens(FILE *fd, char *dirnow)
 
   // Begin by setting v:this_session, and then other sessionable variables.
   PUTLINE_FAIL("let v:this_session=expand(\"<sfile>:p\")");
+
+  PUTLINE_FAIL("doautoall SessionLoadPre");
+
   if (ssop_flags & kOptSsopFlagGlobals) {
     if (store_session_globals(fd) == FAIL) {
       return FAIL;

test/old/testdir/test_autocmd.vim

@@ -1135,6 +1135,71 @@ func Test_BufEnter()
   only
 endfunc
 
+func Test_autocmd_SessLoadPre()
+  tabnew
+  set noswapfile
+  mksession! Session.vim
+
+  call assert_false(exists('g:session_loaded_var'))
+
+  let content =<< trim [CODE]
+    set nocp noswapfile
+
+    func! Assert(cond, msg)
+      if !a:cond
+        echomsg "ASSERT_FAIL: " .. a:msg
+      else
+        echomsg "ASSERT_OK: " .. a:msg
+      endif
+    endfunc
+
+    func! OnSessionLoadPre()
+      call Assert(!exists('g:session_loaded_var'),
+            \ 'SessionLoadPre: var NOT set')
+    endfunc
+    au SessionLoadPre * call OnSessionLoadPre()
+
+    func! OnSessionLoadPost()
+      call Assert(exists('g:session_loaded_var'),
+            \ 'SessionLoadPost: var IS set')
+      echomsg "SessionLoadPost DONE"
+    endfunc
+    au SessionLoadPost * call OnSessionLoadPost()
+
+    func! WriteErrors()
+      call writefile([execute("messages")], "XerrorsPost")
+    endfunc
+    au VimLeave * call WriteErrors()
+  [CODE]
+
+  call writefile(content, 'Xvimrc', 'D')
+
+  call writefile(
+        \ ['let g:session_loaded_var = 1'],
+        \ 'Sessionx.vim',
+        \ 'b'
+        \ )
+
+  " --- Run child Vim ---
+  call system(
+        \ GetVimCommand('Xvimrc')
+        \ .. ' --headless --noplugins -S Session.vim -c cq'
+        \ )
+
+  call WaitForAssert({-> assert_true(filereadable('XerrorsPost'))})
+
+  let errors = join(readfile('XerrorsPost'), "\n")
+  call assert_notmatch('ASSERT_FAIL', errors)
+  call assert_match('ASSERT_OK: SessionLoadPre: var NOT set', errors)
+  call assert_match('ASSERT_OK: SessionLoadPost: var IS set', errors)
+  call assert_match('SessionLoadPost DONE', errors)
+
+  set swapfile
+  for file in ['Session.vim', 'Sessionx.vim', 'XerrorsPost']
+    call delete(file)
+  endfor
+endfunc
+
 " Closing a window might cause an endless loop
 " E814 for older Vims
 func Test_autocmd_bufwipe_in_SessLoadPost()