depends: Use $(package)_file_name when downloading from the fallback #33580

The server hosting the fallbacks uses make download so the files are only available with their overridden names rather than the original name on the upstream source. We should therefore also use the overridden name when downloading from the fallback.

Fixes https://github.com/bitcoin-core/bitcoincore.org/issues/1168

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/33580.

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utACK 671b774d1b58c491b53f2b2f6ee42fb6b65a0e71. I was going to PR the same change.

For more context: Our sources server runs make download and makes those sources available as a backup host.

We can't control upstream filenames, so sometimes we rename them for local storage in depends. For ex "v0.1.tar.gz" may become "libfoo-v0.1.tar.gz".

But when we try to fetch "v0.1.tar.gz" from our backup, it doesn't exist, because it's been renamed to "libfoo-v0.1.tar.gz" there.

So instead of requesting the original filename from our backup, request the renamed file. If there's a hash mismatch, that'll be detected later on, so there's no loss of security.

ut ACK 671b774d1b58c491b53f2b2f6ee42fb6b65a0e71

PR changes the third argument of fetch_file_inner() in the fallback scenario. This change ensures that, in the fallback scenario, the "friendly filename" is used to download the file instead of the "filename."

The file contains only the download logic, and I haven’t been able to verify whether the "friendly filename" is constructed the same way for the upload to the fallback server.

<details><summary>code analysis</summary>

The code is somewhat confusing to read because of double use of $(4). Once as the fourth argument to fetch_file() and once as the fourth arguments to fetch_file_inner(). Where in the fallback scenario the fourth argument of fetch_file() is used as both the 3rd and 4th argument of fetch_file_inner()

The 4th argument of fetch_file() is $($(1)_file_name) (line129) which is the "friendly filename name"

But we can conclude that in the fallback scenario the "friendly file name" is also used to download from the fallback server.

</details>

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Guix builds (on x86_64) [untrusted test-only build, possibly unsafe, not for production use]

ACK 671b774d1b58c491b53f2b2f6ee42fb6b65a0e71, tested with the following patch:

--- a/depends/packages/native_capnp.mk
+++ b/depends/packages/native_capnp.mk
@@ -1,6 +1,6 @@
 package=native_capnp
 $(package)_version=1.2.0
-$(package)_download_path=https://capnproto.org/
+$(package)_download_path=https://capnproto.org.FAIL/
 $(package)_download_file=capnproto-c++-$($(package)_version).tar.gz
 $(package)_file_name=capnproto-cxx-$($(package)_version).tar.gz
 $(package)_sha256_hash=ed00e44ecbbda5186bc78a41ba64a8dc4a861b5f8d4e822959b0144ae6fd42ef

On the master branch:

$ gmake -C depends clean-all
$ gmake -C depends capnp_fetched
gmake: Entering directory '/home/hebasto/dev/bitcoin/depends'
Fetching capnproto-c++-1.2.0.tar.gz from https://capnproto.org.FAIL/
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Warning: Problem : timeout. Will retry in 1 seconds. 3 retries left.
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Warning: Problem : timeout. Will retry in 2 seconds. 2 retries left.
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Warning: Problem : timeout. Will retry in 4 seconds. 1 retries left.
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Fetching capnproto-c++-1.2.0.tar.gz from https://bitcoincore.org/depends-sources
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
curl: (22) The requested URL returned error: 404
gmake: *** [funcs.mk:343: /home/hebasto/dev/bitcoin/depends/sources/download-stamps/.stamp_fetched-capnp-capnproto-cxx-1.2.0.tar.gz.hash] Error 22
gmake: Leaving directory '/home/hebasto/dev/bitcoin/depends'

This PR:

$ gmake -C depends clean-all
$ gmake -C depends capnp_fetched
gmake: Entering directory '/home/hebasto/dev/bitcoin/depends'
Fetching capnproto-c++-1.2.0.tar.gz from https://capnproto.org.FAIL/
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Warning: Problem : timeout. Will retry in 1 seconds. 3 retries left.
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Warning: Problem : timeout. Will retry in 2 seconds. 2 retries left.
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Warning: Problem : timeout. Will retry in 4 seconds. 1 retries left.
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: capnproto.org.FAIL
Fetching capnproto-cxx-1.2.0.tar.gz from https://bitcoincore.org/depends-sources
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1727k  100 1727k    0     0   739k      0  0:00:02  0:00:02 --:--:--  739k
/home/hebasto/dev/bitcoin/depends/work/download/capnp-1.2.0/capnproto-cxx-1.2.0.tar.gz.temp: OK
gmake: Leaving directory '/home/hebasto/dev/bitcoin/depends'

Backported to 30.x in #33609.