Asset caching using a script does not work

[jonathanverner]

Describe the bug When trying to use the asset caching tweaking e.g. the instructions from Creating a x-script Asset Caching source for NuGet to my usecase (using a python script), vcpkg fails when trying to execute the script.

On Windows, I just get

Downloading 7zip...
https://github.com/ip7z/7zip/releases/download/23.01/7z2301-extra.7z->C:\tools\vcpkg\downloads\7z2301-extra.7z
error: Failed to download from mirror set
error: Launching <mirror-script>: error: calling CreateProcessW failed with 2 (The system cannot find the file specified.)
linux I get a more verbose output, the relevant part of which:

on Linux the message is slightly more verbose

[DEBUG] 1000: execute_process( "/usr/bin/python3 /builds/team-b/crescendomd/tools/nexus/cache_vcpkg_assets.py https://github.com/catchorg/Catch2/archive/v3.3.2.tar.gz 3d0c5666509a19be54ea0c48a3c8e1c4a951a2d991a7c9f7fe6d326661464538f1ab9dc573b1b2647f49fb6bef45bbd866142a4ce0fba38545ad182b8d55f61f /tools/vcpkg/downloads/catchorg-Catch2-v3.3.2.tar.gz.263.part")
sh: 1: /usr/bin/python3 /builds/team-b/crescendomd/tools/nexus/cache_vcpkg_assets.py https://github.com/catchorg/Catch2/archive/v3.3.2.tar.gz 3d0c5666509a19be54ea0c48a3c8e1c4a951a2d991a7c9f7fe6d326661464538f1ab9dc573b1b2647f49fb6bef45bbd866142a4ce0fba38545ad182b8d55f61f /tools/vcpkg/downloads/catchorg-Catch2-v3.3.2.tar.gz.263.part: not found
[DEBUG] 1000: cmd_execute_and_stream_data() returned 127 after      808 us
error: Failed to download from mirror set
error: <mirror-script> failed with exit code: (127).
sh: 1: /usr/bin/python3 /builds/team-b/crescendomd/tools/nexus/cache_vcpkg_assets.py https://github.com/catchorg/Catch2/archive/v3.3.2.tar.gz 3d0c5666509a19be54ea0c48a3c8e1c4a951a2d991a7c9f7fe6d326661464538f1ab9dc573b1b2647f49fb6bef45bbd866142a4ce0fba38545ad182b8d55f61f /tools/vcpkg/downloads/catchorg-Catch2-v3.3.2.tar.gz.263.part: not found
[DEBUG] /mnt/vss/_work/1/s/src/vcpkg/base/downloads.cpp(1029): 

Note that the variable X_VCPKG_ASSET_SOURCES is set to

x-script,C:/Python311/python.exe C:\builds\team-b\crescendomd/tools/nexus/cache_vcpkg_assets.py {url} {sha512} {dst};x-block-origin

As far as I can tell, it seems that vcpkg takes the X_VCPKG_ASSET_SOURCES variable, replaces the placeholders with data and passes the result directly to CreateProcessW or sh as a single argument. So, e.g., sh tries looking for an executable called /usr/bin/python3 /builds/team-b/crescendomd/tools/nexus/cache_vcpkg_assets.py https://github.com/catchorg/Catch2/archive/v3.3.2.tar.gz 3d0c5666509a19be54ea0c48a3c8e1c4a951a2d991a7c9f7fe6d326661464538f1ab9dc573b1b2647f49fb6bef45bbd866142a4ce0fba38545ad182b8d55f61f /tools/vcpkg/downloads/catchorg-Catch2-v3.3.2.tar.gz.263.part instead of looking for /usr/bin/python3 and passing the rest as commandline arguments.

This will obviously fail with file not found error.

[jonathanverner]

I can easily reproduce it on version 2024-01-12. The issue seems to be fixed on master.

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