diff --git a/src/nvim/ex_cmds2.c b/src/nvim/ex_cmds2.c
index 5c245d8262..6542ab41f5 100644
--- a/src/nvim/ex_cmds2.c
+++ b/src/nvim/ex_cmds2.c
@@ -1860,72 +1860,11 @@ static bool concat_continued_line(garray_T *const ga, const int init_growsize,
   return true;
 }
 
-/// Concatenate (possibly many) VimL lines starting with line continuations into
-/// a growarray. @see concat_continued_line
-///
-/// @note All parameters, excluding `ga`, accept expressions that are evaluated
-///       once for each line; side-effects may be triggered many times!
-///
-/// @param ga  the growarray to append to
-/// @param cond  should evaluate to true if a next line exists
-/// @param line  should evaluate to the current line
-/// @param next  should handle fetching the next line when evaluated
-#define CONCAT_CONTINUED_LINES(ga, cond, line, next) \
-  do { \
-    garray_T *const ga_ = (ga); \
-    const int init_growsize_ = ga_->ga_growsize; \
-    for (; (cond); (next)) { \
-      const char_u *const line_ = (line); \
-      if (!concat_continued_line(ga_, init_growsize_, line_, STRLEN(line_))) { \
-        break; \
-      } \
-    } \
-  } while (false)
-
 typedef struct {
   linenr_T curr_lnum;
   const linenr_T final_lnum;
 } GetBufferLineCookie;
 
-/// Get one line from the current selection in the buffer.
-/// Called by do_cmdline() when it's called from cmd_source_buffer().
-///
-/// @return pointer to allocated line, or NULL for end-of-file or
-///         some error.
-static char_u *get_buffer_line(int c, void *cookie, int indent, bool do_concat)
-{
-  GetBufferLineCookie *p = cookie;
-  if (p->curr_lnum > p->final_lnum) {
-    return NULL;
-  }
-  garray_T ga;
-  ga_init(&ga, sizeof(char_u), 400);
-  ga_concat(&ga, ml_get(p->curr_lnum++));
-  if (do_concat && vim_strchr(p_cpo, CPO_CONCAT) == NULL) {
-    CONCAT_CONTINUED_LINES(&ga, p->curr_lnum <= p->final_lnum,
-                           ml_get(p->curr_lnum), p->curr_lnum++);
-  }
-  ga_append(&ga, NUL);
-  return ga.ga_data;
-}
-
-static void cmd_source_buffer(const exarg_T *eap)
-  FUNC_ATTR_NONNULL_ALL
-{
-  GetBufferLineCookie cookie = {
-      .curr_lnum = eap->line1,
-      .final_lnum = eap->line2,
-  };
-  if (curbuf != NULL && curbuf->b_fname
-      && path_with_extension((const char *)curbuf->b_fname, "lua")) {
-    nlua_source_using_linegetter(get_buffer_line, (void *)&cookie,
-                                 ":source (no file)");
-  } else {
-    source_using_linegetter((void *)&cookie, get_buffer_line,
-                            ":source (no file)");
-  }
-}
-
 /// ":source" and associated commands.
 ///
 /// @return address holding the next breakpoint line for a source cookie
@@ -2033,6 +1972,40 @@ static int source_using_linegetter(void *cookie,
   return retval;
 }
 
+static void cmd_source_buffer(const exarg_T *const eap)
+  FUNC_ATTR_NONNULL_ALL
+{
+  if (curbuf == NULL) {
+    return;
+  }
+  garray_T ga;
+  ga_init(&ga, sizeof(char_u), 400);
+  const linenr_T final_lnum = eap->line2;
+  // Copy the contents to be executed.
+  for (linenr_T curr_lnum = eap->line1; curr_lnum <= final_lnum; curr_lnum++) {
+    // Adjust growsize to current length to speed up concatenating many lines.
+    if (ga.ga_len > 400) {
+      ga_set_growsize(&ga, MAX(ga.ga_len, 8000));
+    }
+    ga_concat(&ga, ml_get(curr_lnum));
+    ga_append(&ga, NL);
+  }
+  ((char_u *)ga.ga_data)[ga.ga_len - 1] = NUL;
+  const GetStrLineCookie cookie = {
+      .buf = ga.ga_data,
+      .offset = 0,
+  };
+  if (curbuf->b_fname
+      && path_with_extension((const char *)curbuf->b_fname, "lua")) {
+    nlua_source_using_linegetter(get_str_line, (void *)&cookie,
+                                 ":source (no file)");
+  } else {
+    source_using_linegetter((void *)&cookie, get_str_line,
+                            ":source (no file)");
+  }
+  ga_clear(&ga);
+}
+
 /// Executes lines in `src` as Ex commands.
 ///
 /// @see do_source()
@@ -2490,9 +2463,12 @@ char_u *getsourceline(int c, void *cookie, int indent, bool do_concat)
 
       ga_init(&ga, (int)sizeof(char_u), 400);
       ga_concat(&ga, line);
-      CONCAT_CONTINUED_LINES(&ga, sp->nextline != NULL, sp->nextline,
-                             (xfree(sp->nextline),
-                              sp->nextline = get_one_sourceline(sp)));
+      while (sp->nextline != NULL
+             && concat_continued_line(&ga, 400, sp->nextline,
+                                      STRLEN(sp->nextline))) {
+        xfree(sp->nextline);
+        sp->nextline = get_one_sourceline(sp);
+      }
       ga_append(&ga, NUL);
       xfree(line);
       line = ga.ga_data;
diff --git a/test/functional/ex_cmds/source_spec.lua b/test/functional/ex_cmds/source_spec.lua
index 3cef412bb6..bdf6ae76d1 100644
--- a/test/functional/ex_cmds/source_spec.lua
+++ b/test/functional/ex_cmds/source_spec.lua
@@ -60,6 +60,15 @@ describe(':source', function()
     eq('Vim(let):E15: Invalid expression: #{', exc_exec("'<,'>source"))
   end)
 
+  it('does not break if current buffer is modified while sourced', function()
+    insert [[
+      bwipeout!
+      let a = 123
+    ]]
+    command('source')
+    eq('123', meths.exec('echo a', true))
+  end)
+
   it('multiline heredoc command', function()
     insert([[
       lua << EOF