feat(lua-api): avoid unnecessary allocations (#19877)

Lua makes (or reuses) an internal copy of strings, so we can safely push
buf pointers onto the stack.

5 files changed, 130 insertions, 74 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 51fedb302a..29c2ed6028 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -271,6 +271,7 @@ ArrayOf(String) nvim_buf_get_lines(uint64_t channel_id,
                                    Integer start,
                                    Integer end,
                                    Boolean strict_indexing,
+                                   lua_State *lstate,
                                    Error *err)
   FUNC_API_SINCE(1)
 {
@@ -300,21 +301,18 @@ ArrayOf(String) nvim_buf_get_lines(uint64_t channel_id,
     return rv;
   }
 
-  rv.size = (size_t)(end - start);
-  rv.items = xcalloc(rv.size, sizeof(Object));
+  size_t size = (size_t)(end - start);
 
-  if (!buf_collect_lines(buf, rv.size, start,
-                         (channel_id != VIML_INTERNAL_CALL), &rv, err)) {
+  init_line_array(lstate, &rv, size);
+
+  if (!buf_collect_lines(buf, size, (linenr_T)start, (channel_id != VIML_INTERNAL_CALL), &rv,
+                         lstate, err)) {
     goto end;
   }
 
 end:
   if (ERROR_SET(err)) {
-    for (size_t i = 0; i < rv.size; i++) {
-      xfree(rv.items[i].data.string.data);
-    }
-
-    xfree(rv.items);
+    api_free_array(rv);
     rv.items = NULL;
   }
 
@@ -790,7 +788,8 @@ early_end:
 ArrayOf(String) nvim_buf_get_text(uint64_t channel_id, Buffer buffer,
                                   Integer start_row, Integer start_col,
                                   Integer end_row, Integer end_col,
-                                  Dictionary opts, Error *err)
+                                  Dictionary opts, lua_State *lstate,
+                                  Error *err)
   FUNC_API_SINCE(9)
 {
   Array rv = ARRAY_DICT_INIT;
@@ -830,33 +829,38 @@ ArrayOf(String) nvim_buf_get_text(uint64_t channel_id, Buffer buffer,
 
   bool replace_nl = (channel_id != VIML_INTERNAL_CALL);
 
+  size_t size = (size_t)(end_row - start_row) + 1;
+
+  init_line_array(lstate, &rv, size);
+
   if (start_row == end_row) {
-    String line = buf_get_text(buf, start_row, start_col, end_col, replace_nl, err);
+    String line = buf_get_text(buf, start_row, start_col, end_col, err);
     if (ERROR_SET(err)) {
-      return rv;
+      goto end;
     }
-
-    ADD(rv, STRING_OBJ(line));
+    push_linestr(lstate, &rv, line.data, line.size, 0, replace_nl);
     return rv;
   }
 
-  rv.size = (size_t)(end_row - start_row) + 1;
-  rv.items = xcalloc(rv.size, sizeof(Object));
+  String str = buf_get_text(buf, start_row, start_col, MAXCOL - 1, err);
+
+  push_linestr(lstate, &rv, str.data, str.size, 0, replace_nl);
 
-  rv.items[0] = STRING_OBJ(buf_get_text(buf, start_row, start_col, MAXCOL - 1, replace_nl, err));
   if (ERROR_SET(err)) {
     goto end;
   }
 
-  if (rv.size > 2) {
+  if (size > 2) {
     Array tmp = ARRAY_DICT_INIT;
     tmp.items = &rv.items[1];
-    if (!buf_collect_lines(buf, rv.size - 2, start_row + 1, replace_nl, &tmp, err)) {
+    if (!buf_collect_lines(buf, size - 2, (linenr_T)start_row + 1, replace_nl, &tmp, lstate, err)) {
       goto end;
     }
   }
 
-  rv.items[rv.size - 1] = STRING_OBJ(buf_get_text(buf, end_row, 0, end_col, replace_nl, err));
+  str = buf_get_text(buf, end_row, 0, end_col, err);
+  push_linestr(lstate, &rv, str.data, str.size, (int)(size - 1), replace_nl);
+
   if (ERROR_SET(err)) {
     goto end;
   }
@@ -1390,3 +1394,90 @@ static int64_t normalize_index(buf_T *buf, int64_t index, bool end_exclusive, bo
   index++;
   return index;
 }
+
+/// Initialise a string array either:
+/// - on the Lua stack (as a table) (if lstate is not NULL)
+/// - as an API array object (if lstate is NULL).
+///
+/// @param lstate  Lua state. When NULL the Array is initialized instead.
+/// @param a       Array to initialize
+/// @param size    Size of array
+static inline void init_line_array(lua_State *lstate, Array *a, size_t size)
+{
+  if (lstate) {
+    lua_createtable(lstate, (int)size, 0);
+  } else {
+    a->size = size;
+    a->items = xcalloc(a->size, sizeof(Object));
+  }
+}
+
+/// Push a string onto either the Lua stack (as a table element) or an API array object.
+///
+/// For Lua, a table of the correct size must be created first.
+/// API array objects must be pre allocated.
+///
+/// @param lstate      Lua state. When NULL the Array is pushed to instead.
+/// @param a           Array to push onto when not using Lua
+/// @param s           String to push
+/// @param len         Size of string
+/// @param idx         0-based index to place s
+/// @param replace_nl  Replace newlines ('\n') with null ('\0')
+static void push_linestr(lua_State *lstate, Array *a, const char *s, size_t len, int idx,
+                         bool replace_nl)
+{
+  if (lstate) {
+    // Vim represents NULs as NLs
+    if (s && replace_nl && strchr(s, '\n')) {
+      char *tmp = xmemdupz(s, len);
+      strchrsub(tmp, '\n', '\0');
+      lua_pushlstring(lstate, tmp, len);
+      xfree(tmp);
+    } else {
+      lua_pushlstring(lstate, s, len);
+    }
+    lua_rawseti(lstate, -2, idx + 1);
+  } else {
+    String str = STRING_INIT;
+    if (s) {
+      str = cbuf_to_string(s, len);
+      if (replace_nl) {
+        // Vim represents NULs as NLs, but this may confuse clients.
+        strchrsub(str.data, '\n', '\0');
+      }
+    }
+
+    a->items[idx] = STRING_OBJ(str);
+  }
+}
+
+/// Collects `n` buffer lines into array `l` and/or lua_State `lstate`, optionally replacing
+/// newlines with NUL.
+///
+/// @param buf Buffer to get lines from
+/// @param n Number of lines to collect
+/// @param replace_nl Replace newlines ("\n") with NUL
+/// @param start Line number to start from
+/// @param[out] l If not NULL, Lines are copied here
+/// @param[out] lstate If not NULL, Lines are pushed into a table onto the stack
+/// @param err[out] Error, if any
+/// @return true unless `err` was set
+bool buf_collect_lines(buf_T *buf, size_t n, linenr_T start, bool replace_nl, Array *l,
+                       lua_State *lstate, Error *err)
+{
+  for (size_t i = 0; i < n; i++) {
+    linenr_T lnum = start + (linenr_T)i;
+
+    if (lnum >= MAXLNUM) {
+      if (err != NULL) {
+        api_set_error(err, kErrorTypeValidation, "Line index is too high");
+      }
+      return false;
+    }
+
+    char *bufstr = ml_get_buf(buf, lnum, false);
+    push_linestr(lstate, l, bufstr, strlen(bufstr), (int)i, replace_nl);
+  }
+
+  return true;
+}
diff --git a/src/nvim/api/buffer.h b/src/nvim/api/buffer.h
index 1c4a93a587..0814da63cd 100644
--- a/src/nvim/api/buffer.h
+++ b/src/nvim/api/buffer.h
@@ -1,7 +1,10 @@
 #ifndef NVIM_API_BUFFER_H
 #define NVIM_API_BUFFER_H
 
+#include <lauxlib.h>
+
 #include "nvim/api/private/defs.h"
+#include "nvim/buffer_defs.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/buffer.h.generated.h"
diff --git a/src/nvim/api/deprecated.c b/src/nvim/api/deprecated.c
index abaac07755..8e1a615bbb 100644
--- a/src/nvim/api/deprecated.c
+++ b/src/nvim/api/deprecated.c
@@ -190,7 +190,7 @@ String buffer_get_line(Buffer buffer, Integer index, Error *err)
   String rv = { .size = 0 };
 
   index = convert_index(index);
-  Array slice = nvim_buf_get_lines(0, buffer, index, index + 1, true, err);
+  Array slice = nvim_buf_get_lines(0, buffer, index, index + 1, true, NULL, err);
 
   if (!ERROR_SET(err) && slice.size) {
     rv = slice.items[0].data.string;
@@ -263,7 +263,7 @@ ArrayOf(String) buffer_get_line_slice(Buffer buffer,
 {
   start = convert_index(start) + !include_start;
   end = convert_index(end) + include_end;
-  return nvim_buf_get_lines(0, buffer, start, end, false, err);
+  return nvim_buf_get_lines(0, buffer, start, end, false, NULL, err);
 }
 
 /// Replaces a line range on the buffer
diff --git a/src/nvim/api/options.c b/src/nvim/api/options.c
index 92f20fff48..1b04392d47 100644
--- a/src/nvim/api/options.c
+++ b/src/nvim/api/options.c
@@ -256,7 +256,7 @@ void nvim_set_option(uint64_t channel_id, String name, Object value, Error *err)
 /// @param name     Option name
 /// @param[out] err Error details, if any
 /// @return         Option value (global)
-Object nvim_get_option(String name, Error *err)
+Object nvim_get_option(String name, Arena *arena, Error *err)
   FUNC_API_SINCE(1)
 {
   return get_option_from(NULL, SREQ_GLOBAL, name, err);
@@ -268,7 +268,7 @@ Object nvim_get_option(String name, Error *err)
 /// @param name       Option name
 /// @param[out] err   Error details, if any
 /// @return Option value
-Object nvim_buf_get_option(Buffer buffer, String name, Error *err)
+Object nvim_buf_get_option(Buffer buffer, String name, Arena *arena, Error *err)
   FUNC_API_SINCE(1)
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
@@ -306,7 +306,7 @@ void nvim_buf_set_option(uint64_t channel_id, Buffer buffer, String name, Object
 /// @param name     Option name
 /// @param[out] err Error details, if any
 /// @return Option value
-Object nvim_win_get_option(Window window, String name, Error *err)
+Object nvim_win_get_option(Window window, String name, Arena *arena, Error *err)
   FUNC_API_SINCE(1)
 {
   win_T *win = find_window_by_handle(window, err);
@@ -346,7 +346,7 @@ void nvim_win_set_option(uint64_t channel_id, Window window, String name, Object
 /// @param name The option name
 /// @param[out] err Details of an error that may have occurred
 /// @return the option value
-Object get_option_from(void *from, int type, String name, Error *err)
+static Object get_option_from(void *from, int type, String name, Error *err)
 {
   Object rv = OBJECT_INIT;
 
@@ -358,8 +358,7 @@ Object get_option_from(void *from, int type, String name, Error *err)
   // Return values
   int64_t numval;
   char *stringval = NULL;
-  int flags = get_option_value_strict(name.data, &numval, &stringval,
-                                      type, from);
+  int flags = get_option_value_strict(name.data, &numval, &stringval, type, from);
 
   if (!flags) {
     api_set_error(err, kErrorTypeValidation, "Invalid option name: '%s'",
diff --git a/src/nvim/api/private/helpers.c b/src/nvim/api/private/helpers.c
index 73b5489d5c..d10d17c88d 100644
--- a/src/nvim/api/private/helpers.c
+++ b/src/nvim/api/private/helpers.c
@@ -394,6 +394,12 @@ String cstrn_to_string(const char *str, size_t maxsize)
   return cbuf_to_string(str, STRNLEN(str, maxsize));
 }
 
+String cstrn_as_string(char *str, size_t maxsize)
+  FUNC_ATTR_NONNULL_ALL
+{
+  return cbuf_as_string(str, STRNLEN(str, maxsize));
+}
+
 /// Creates a String using the given C string. Unlike
 /// cstr_to_string this function DOES NOT copy the C string.
 ///
@@ -462,53 +468,15 @@ Array string_to_array(const String input, bool crlf)
   return ret;
 }
 
-/// Collects `n` buffer lines into array `l`, optionally replacing newlines
-/// with NUL.
-///
-/// @param buf Buffer to get lines from
-/// @param n Number of lines to collect
-/// @param replace_nl Replace newlines ("\n") with NUL
-/// @param start Line number to start from
-/// @param[out] l Lines are copied here
-/// @param err[out] Error, if any
-/// @return true unless `err` was set
-bool buf_collect_lines(buf_T *buf, size_t n, int64_t start, bool replace_nl, Array *l, Error *err)
-{
-  for (size_t i = 0; i < n; i++) {
-    int64_t lnum = start + (int64_t)i;
-
-    if (lnum >= MAXLNUM) {
-      if (err != NULL) {
-        api_set_error(err, kErrorTypeValidation, "Line index is too high");
-      }
-      return false;
-    }
-
-    const char *bufstr = ml_get_buf(buf, (linenr_T)lnum, false);
-    Object str = STRING_OBJ(cstr_to_string(bufstr));
-
-    if (replace_nl) {
-      // Vim represents NULs as NLs, but this may confuse clients.
-      strchrsub(str.data.string.data, '\n', '\0');
-    }
-
-    l->items[i] = str;
-  }
-
-  return true;
-}
-
 /// Returns a substring of a buffer line
 ///
 /// @param buf          Buffer handle
 /// @param lnum         Line number (1-based)
 /// @param start_col    Starting byte offset into line (0-based)
 /// @param end_col      Ending byte offset into line (0-based, exclusive)
-/// @param replace_nl   Replace newlines ('\n') with null ('\0')
 /// @param err          Error object
 /// @return The text between start_col and end_col on line lnum of buffer buf
-String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col, bool replace_nl,
-                    Error *err)
+String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col, Error *err)
 {
   String rv = STRING_INIT;
 
@@ -517,7 +485,7 @@ String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col
     return rv;
   }
 
-  const char *bufstr = ml_get_buf(buf, (linenr_T)lnum, false);
+  char *bufstr = ml_get_buf(buf, (linenr_T)lnum, false);
   size_t line_length = strlen(bufstr);
 
   start_col = start_col < 0 ? (int64_t)line_length + start_col + 1 : start_col;
@@ -537,12 +505,7 @@ String buf_get_text(buf_T *buf, int64_t lnum, int64_t start_col, int64_t end_col
     return rv;
   }
 
-  rv = cstrn_to_string(&bufstr[start_col], (size_t)(end_col - start_col));
-  if (replace_nl) {
-    strchrsub(rv.data, '\n', '\0');
-  }
-
-  return rv;
+  return cstrn_as_string((char *)&bufstr[start_col], (size_t)(end_col - start_col));
 }
 
 void api_free_string(String value)