setopt() uses SvPV(), doesn’t document whether it’s for bytes or characters

[FGasper]

> perl -Mstrict -w -e'use Net::Curl::Easy qw(:constants); my $payload = "épée"; my $easy = Net::Curl::Easy->new(); $easy->setopt( +CURLOPT_URL, "http://example.com" ); $easy->setopt( CURLOPT_VERBOSE, 1 ); $easy->setopt( +CURLOPT_COPYPOSTFIELDS, $payload ); $easy->perform()' >/dev/null
* About to connect() to example.com port 80 (#0)
*   Trying 2606:2800:220:1:248:1893:25c8:1946...
* Connected to example.com (2606:2800:220:1:248:1893:25c8:1946) port 80 (#0)
> POST / HTTP/1.1
Host: example.com
Accept: */*
Content-Length: 6
Content-Type: application/x-www-form-urlencoded

* upload completely sent off: 6 out of 6 bytes
<snip>

Note that the upload size is 6 bytes.

But if you change the SV’s internal representation, the output also changes:

> perl -MJSON -Mstrict -w -e'use Net::Curl::Easy qw(:constants); my $payload = "épée"; $payload = JSON::decode_json( JSON::encode_json([$payload]) )->[0]; my $easy = Net::Curl::Easy->new(); $easy->setopt( +CURLOPT_URL, "http://example.com" ); $easy->setopt( CURLOPT_VERBOSE, 1 ); $easy->setopt( +CURLOPT_COPYPOSTFIELDS, $payload ); $easy->perform()' >/dev/null
* About to connect() to example.com port 80 (#0)
*   Trying 2606:2800:220:1:248:1893:25c8:1946...
* Connected to example.com (2606:2800:220:1:248:1893:25c8:1946) port 80 (#0)
> POST / HTTP/1.1
Host: example.com
Accept: */*
Content-Length: 10
Content-Type: application/x-www-form-urlencoded

* upload completely sent off: 10 out of 10 bytes
<snip>

Pure Perl code isn’t supposed to have to care about a string’s internal representation, but this behaviour of Net::Curl forces it.

The proper fix is probably to create a setopt_bytes that always treats the given SV as a byte string. (So if it’s upgraded, what curl gets from Perl will be the downgraded variant.)

Additionally, the documentation should be updated to advise of this situation with plain setopt().

A workaround is to call utf8::downgrade() before giving the value to setopt().