4 files changed, 77 insertions, 22 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 698172442e..c2f84735b2 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -5859,27 +5859,57 @@ write_blob_error:
   return false;
 }
 
-/// Read a blob from a file `fd`.
+/// Read blob from file "fd".
+/// Caller has allocated a blob in "rettv".
 ///
 /// @param[in]  fd  File to read from.
-/// @param[in,out]  blob  Blob to write to.
+/// @param[in,out]  rettv  Blob to write to.
+/// @param[in]  offset  Read the file from the specified offset.
+/// @param[in]  size  Read the specified size, or -1 if no limit.
 ///
-/// @return true on success, or false on failure.
-bool read_blob(FILE *const fd, blob_T *const blob)
+/// @return  OK on success, or FAIL on failure.
+int read_blob(FILE *const fd, typval_T *rettv, off_T offset, off_T size_arg)
   FUNC_ATTR_NONNULL_ALL
 {
+  blob_T *const blob = rettv->vval.v_blob;
   FileInfo file_info;
   if (!os_fileinfo_fd(fileno(fd), &file_info)) {
-    return false;
+    return FAIL;  // can't read the file, error
+  }
+
+  int whence;
+  off_T size = size_arg;
+  if (offset >= 0) {
+    if (size == -1) {
+      // size may become negative, checked below
+      size = (off_T)os_fileinfo_size(&file_info) - offset;
+    }
+    whence = SEEK_SET;
+  } else {
+    if (size == -1) {
+      size = -offset;
+    }
+    whence = SEEK_END;
+  }
+  // Trying to read bytes that aren't there results in an empty blob, not an
+  // error.
+  if (size < 0 || size > (off_T)os_fileinfo_size(&file_info)) {
+    return OK;
+  }
+  if (vim_fseek(fd, offset, whence) != 0) {
+    return OK;
   }
-  const int size = (int)os_fileinfo_size(&file_info);
-  ga_grow(&blob->bv_ga, size);
-  blob->bv_ga.ga_len = size;
+
+  ga_grow(&blob->bv_ga, (int)size);
+  blob->bv_ga.ga_len = (int)size;
   if (fread(blob->bv_ga.ga_data, 1, (size_t)blob->bv_ga.ga_len, fd)
       < (size_t)blob->bv_ga.ga_len) {
-    return false;
+    // An empty blob is returned on error.
+    tv_blob_free(rettv->vval.v_blob);
+    rettv->vval.v_blob = NULL;
+    return FAIL;
   }
-  return true;
+  return OK;
 }
 
 /// Saves a typval_T as a string.
diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index a476e20339..24263d9aee 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -296,7 +296,7 @@ return {
     perleval={args=1, base=1},
     rand={args={0, 1}, base=1},
     range={args={1, 3}, base=1},
-    readblob={args=1, base=1},
+    readblob={args={1, 3}, base=1},
     readdir={args={1, 2}, base=1},
     readfile={args={1, 3}, base=1},
     reduce={args={2, 3}, base=1},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index d3bac14754..5da6233e38 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -5592,15 +5592,24 @@ static void read_file_or_blob(typval_T *argvars, typval_T *rettv, bool always_bl
   ptrdiff_t prevlen  = 0;               // length of data in prev
   ptrdiff_t prevsize = 0;               // size of prev buffer
   int64_t maxline  = MAXLNUM;
+  off_T offset = 0;
+  off_T size = -1;
 
   if (argvars[1].v_type != VAR_UNKNOWN) {
-    if (strcmp(tv_get_string(&argvars[1]), "b") == 0) {
-      binary = true;
-    } else if (strcmp(tv_get_string(&argvars[1]), "B") == 0) {
-      blob = true;
-    }
-    if (argvars[2].v_type != VAR_UNKNOWN) {
-      maxline = tv_get_number(&argvars[2]);
+    if (always_blob) {
+      offset = (off_T)tv_get_number(&argvars[1]);
+      if (argvars[2].v_type != VAR_UNKNOWN) {
+        size = (off_T)tv_get_number(&argvars[2]);
+      }
+    } else {
+      if (strcmp(tv_get_string(&argvars[1]), "b") == 0) {
+        binary = true;
+      } else if (strcmp(tv_get_string(&argvars[1]), "B") == 0) {
+        blob = true;
+      }
+      if (argvars[2].v_type != VAR_UNKNOWN) {
+        maxline = tv_get_number(&argvars[2]);
+      }
     }
   }
 
@@ -5619,11 +5628,8 @@ static void read_file_or_blob(typval_T *argvars, typval_T *rettv, bool always_bl
 
   if (blob) {
     tv_blob_alloc_ret(rettv);
-    if (!read_blob(fd, rettv->vval.v_blob)) {
+    if (read_blob(fd, rettv, offset, size) == FAIL) {
       semsg(_(e_notread), fname);
-      // An empty blob is returned on error.
-      tv_blob_free(rettv->vval.v_blob);
-      rettv->vval.v_blob = NULL;
     }
     fclose(fd);
     return;
diff --git a/src/nvim/testdir/test_blob.vim b/src/nvim/testdir/test_blob.vim
index b1859f9dfe..dd37fb2f11 100644
--- a/src/nvim/testdir/test_blob.vim
+++ b/src/nvim/testdir/test_blob.vim
@@ -439,10 +439,29 @@ func Test_blob_read_write()
       call writefile(b, 'Xblob')
       VAR br = readfile('Xblob', 'B')
       call assert_equal(b, br)
+      VAR br2 = readblob('Xblob')
+      call assert_equal(b, br2)
+      VAR br3 = readblob('Xblob', 1)
+      call assert_equal(b[1 :], br3)
+      VAR br4 = readblob('Xblob', 1, 2)
+      call assert_equal(b[1 : 2], br4)
+      VAR br5 = readblob('Xblob', -3)
+      call assert_equal(b[-3 :], br5)
+      VAR br6 = readblob('Xblob', -3, 2)
+      call assert_equal(b[-3 : -2], br6)
+
+      VAR br1e = readblob('Xblob', 10000)
+      call assert_equal(0z, br1e)
+      VAR br2e = readblob('Xblob', -10000)
+      call assert_equal(0z, br2e)
+
       call delete('Xblob')
   END
   call CheckLegacyAndVim9Success(lines)
 
+  call assert_fails("call readblob('notexist')", 'E484:')
+  " TODO: How do we test for the E485 error?
+
   " This was crashing when calling readfile() with a directory.
   call assert_fails("call readfile('.', 'B')", 'E17: "." is a directory')
 endfunc