Implement seek function for curl posts

This allows for redirects to be successful in POSTing data from curl
over REST. A new `BufferList::seek(offset, whence)` function was added
to facilitate this.

HISTORY.md

@@ -15,6 +15,7 @@
 * Source built curl only need HTTP support [#1712](https://github.com/TileDB-Inc/TileDB/pull/1712)
 * AWS SDK version bumped to 1.8.6 [#1718](https://github.com/TileDB-Inc/TileDB/pull/1718)
 * Split posix permissions into files and folers permissions [#1719](https://github.com/TileDB-Inc/TileDB/pull/1719)
+* Support seeking for CURL to allow redirects for posting to REST [#1728](https://github.com/TileDB-Inc/TileDB/pull/1728)
 
 ## Deprecations
 

tiledb/sm/buffer/buffer_list.cc

@@ -94,7 +94,10 @@ Status BufferList::read(void* dest, uint64_t nbytes, uint64_t* bytes_read) {
     // Read from buffer
     const uint64_t bytes_in_src = src.size() - current_relative_offset_;
     const uint64_t bytes_from_src = std::min(bytes_in_src, bytes_left);
-    RETURN_NOT_OK(src.read(dest_ptr + dest_offset, bytes_from_src));
+    // If the destination pointer is not null, then read into it
+    // if it is null then we are just seeking
+    if (dest_ptr != nullptr)
+      RETURN_NOT_OK(src.read(dest_ptr + dest_offset, bytes_from_src));
     bytes_left -= bytes_from_src;
     dest_offset += bytes_from_src;
 
@@ -114,6 +117,24 @@ Status BufferList::read(void* dest, uint64_t nbytes, uint64_t* bytes_read) {
   return Status::Ok();
 }
 
+Status BufferList::seek(off_t offset, int whence) {
+  switch (whence) {
+    case SEEK_SET:
+      // We just reset the offsets to 0/start, then fall through to seek_current
+      reset_offset();
+      // fall through
+    case SEEK_CUR:
+      return read(nullptr, offset);
+    case SEEK_END:
+      return Status::BufferError(
+          "SEEK_END operation not supported for BufferList");
+    default:
+      return Status::BufferError("Invalid seek operation for BufferList");
+  }
+
+  return Status::Ok();
+}
+
 void BufferList::reset_offset() {
   offset_ = 0;
   current_buffer_index_ = 0;

tiledb/sm/buffer/buffer_list.h

@@ -82,7 +82,8 @@ class BufferList {
    * Returns an error if the buffers contain less than the requested number of
    * bytes starting from the current offset.
    *
-   * @param dest The buffer to read the data into.
+   * @param dest The buffer to read the data into. If null then will perform
+   * seek.
    * @param nbytes The number of bytes to read.
    * @return Status
    */
@@ -99,6 +100,27 @@ class BufferList {
    */
   Status read_at_most(void* dest, uint64_t nbytes, uint64_t* bytes_read);
 
+  /**
+   * Seek to an offset, similar to lseek or fseek
+   *
+   * Whence options are:
+   *
+   *  SEEK_SET
+   *    The offset is set to offset bytes.
+   *
+   *  SEEK_CUR
+   *    The offset is set to its current location plus offset bytes
+   *
+   *  SEEK_END
+   *    This is not supported. Its purpose in lseek would be to set
+   *    the offset to the size of the BufferList plus offset bytes.
+   *
+   * @param offset Offset to seek to.
+   * @param whence Location to seek from.
+   * @return Status
+   */
+  Status seek(off_t offset, int whence);
+
   /** Resets the current offset for reading. */
   void reset_offset();
 
@@ -121,7 +143,7 @@ class BufferList {
   /**
    * Reads from the current offset into the given destination.
    *
-   * @param dest The buffer to read the data into.
+   * @param dest The buffer to read the data into. If null will perform seek.
    * @param nbytes The number of bytes to read.
    * @param bytes_read Set to the number of bytes actually read.
    * @return Status

tiledb/sm/rest/curl.cc

@@ -161,6 +161,23 @@ size_t buffer_list_read_memory_callback(
   return num_read;
 }
 
+/**
+ * Seek function to handle curl redirects
+ * @param userp user data (buffer list)
+ * @param offset offset to seek to
+ * @param origin whence to seek from
+ * @return SEEKFUNC status
+ */
+static int buffer_list_seek_callback(
+    void* userp, curl_off_t offset, int origin) {
+  BufferList* data = static_cast<BufferList*>(userp);
+  Status status = data->seek(offset, origin);
+  if (status.ok())
+    return CURL_SEEKFUNC_OK;
+
+  return CURL_SEEKFUNC_FAIL;
+}
+
 Curl::Curl()
     : config_(nullptr)
     , curl_(nullptr, curl_easy_cleanup) {
@@ -522,6 +539,10 @@ Status Curl::post_data_common(
   /* pass our list of custom made headers */
   curl_easy_setopt(curl, CURLOPT_HTTPHEADER, *headers);
 
+  /* set seek for handling redirects */
+  curl_easy_setopt(curl, CURLOPT_SEEKFUNCTION, &buffer_list_seek_callback);
+  curl_easy_setopt(curl, CURLOPT_SEEKDATA, data);
+
   return Status::Ok();
 }