tool: determine the correct fopen option for -D

This commit fixes a bug in the dump-header feature regarding the
determination of the second fopen(3) option.

Reported-by: u20221022 on github

See #4753
See #4762
Fixes #10074
Closes #10079

1 files changed, 15 insertions, 1 deletions

diff --git a/src/tool_operate.c b/src/tool_operate.c
index 7af73a266..79db063a5 100644
--- a/src/tool_operate.c
+++ b/src/tool_operate.c
@@ -969,7 +969,21 @@ static CURLcode single_transfer(struct GlobalConfig *global,
           /* open file for output: */
           if(strcmp(config->headerfile, "-")) {
             FILE *newfile;
-            newfile = fopen(config->headerfile, per->prev == NULL?"wb":"ab");
+
+            /*
+             * this checks if the previous transfer had the same
+             * OperationConfig, which would mean, that the an output file has
+             * already been created and data can be appened to it, instead
+             * of overwriting it.
+             * TODO: Consider placing the file handle inside the
+             * OperationConfig, so that it does not need to be opened/closed
+             * for every transfer.
+             */
+            if(per->prev && per->prev->config == config)
+              newfile = fopen(config->headerfile, "ab+");
+            else
+              newfile = fopen(config->headerfile, "wb+");
+
             if(!newfile) {
               warnf(global, "Failed to open %s\n", config->headerfile);
               result = CURLE_WRITE_ERROR;