feat(shada): don't store jumplist if '0 in 'shada'

runtime/doc/options.txt

@@ -5187,8 +5187,8 @@ A jump table for the options with a short description can be found at |Q_op|.
 	'	Maximum number of previously edited files for which the marks
 		are remembered.  This parameter must always be included when
 		'shada' is non-empty.
-		Including this item also means that the |jumplist| and the
+		If non-zero, then the |jumplist| and the |changelist| are also
-		|changelist| are stored in the shada file.
+		stored in the shada file.
 							*shada-/*
 	/	Maximum number of items in the search pattern history to be
 		saved.  If non-zero, then the previous search and substitute

runtime/lua/vim/_meta/options.lua

@@ -5441,8 +5441,8 @@ vim.go.ssop = vim.go.sessionoptions
 --- '	Maximum number of previously edited files for which the marks
 --- 	are remembered.  This parameter must always be included when
 --- 	'shada' is non-empty.
---- 	Including this item also means that the `jumplist` and the
+--- 	If non-zero, then the `jumplist` and the `changelist` are also
---- 	`changelist` are stored in the shada file.
+--- 	stored in the shada file.
 --- 						*shada-/*
 --- /	Maximum number of items in the search pattern history to be
 --- 	saved.  If non-zero, then the previous search and substitute

src/nvim/options.lua

@@ -7296,8 +7296,8 @@ local options = {
         '	Maximum number of previously edited files for which the marks
         	are remembered.  This parameter must always be included when
         	'shada' is non-empty.
-        	Including this item also means that the |jumplist| and the
+        	If non-zero, then the |jumplist| and the |changelist| are also
-        	|changelist| are stored in the shada file.
+        	stored in the shada file.
         						*shada-/*
         /	Maximum number of items in the search pattern history to be
         	saved.  If non-zero, then the previous search and substitute

src/nvim/shada.c

@@ -2396,8 +2396,10 @@ static ShaDaWriteResult shada_write(FileDescriptor *const sd_writer,
     } while (var_iter != NULL);
   }
 
-  // Initialize jump list
+  if (num_marked_files > 0) {  // Skip if '0 in 'shada'
-  wms->jumps_size = shada_init_jumps(wms->jumps, &removable_bufs);
+    // Initialize jump list
+    wms->jumps_size = shada_init_jumps(wms->jumps, &removable_bufs);
+  }
 
   const bool search_highlighted = !(no_hlsearch
                                     || find_shada_parameter('h') != NULL);

test/functional/shada/marks_spec.lua

@@ -152,6 +152,37 @@ describe('ShaDa support code', function()
     eq(saved, exec_capture('jumps'))
   end)
 
+  it("does not dump jumplist if `'0` in shada", function()
+    local empty_jumps = exec_capture('jumps')
+    nvim_command("set shada='0")
+    nvim_command('edit ' .. testfilename_2)
+    nvim_command('normal! G')
+    nvim_command('normal! gg')
+    nvim_command('edit ' .. testfilename)
+    nvim_command('normal! G')
+    nvim_command('normal! gg')
+    nvim_command('enew')
+    nvim_command('normal! gg')
+    expect_exit(nvim_command, 'qall')
+    reset()
+    eq(empty_jumps, exec_capture('jumps'))
+  end)
+
+  it("does read back jumplist even with `'0` in shada", function()
+    nvim_command('edit ' .. testfilename_2)
+    nvim_command('normal! G')
+    nvim_command('normal! gg')
+    nvim_command('edit ' .. testfilename)
+    nvim_command('normal! G')
+    nvim_command('normal! gg')
+    nvim_command('enew')
+    nvim_command('normal! gg')
+    local saved = exec_capture('jumps')
+    expect_exit(nvim_command, 'qall')
+    reset("set shada='0")
+    eq(saved, exec_capture('jumps'))
+  end)
+
   it('when dumping jump list also dumps current position', function()
     nvim_command('edit ' .. testfilename)
     nvim_command('normal! G')