Write detection lacking #20529

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I am trying to start bitcoin-core.daemon via

$ bitcoin-core.daemon -datadir=/home/user/.bitcoin -debug=1

Expected behavior

bitcoin-core.daemon should start. Or if it can't it should provide helpful output what the problem is

Actual behavior

Output is:

Error: Cannot write to data directory '/home/user/.bitcoin'; check permissions.

Looking on the filesystem everything seems fine:

$ ls -lah /home/user/.bitcoin
drwxrwx---  3 user user 4,0K Nov 29 16:44 .
$ mkdir /home/user/.bitcoin/foo
$ echo $?
0

So I took a look, where the error message is actually produced in the source. Digging deeper into DirIsWritable there seems to be a lack of any tracing or debugging. Same with fsbridge - there is no handling of errno. So it seems like the bitcoin binary has no way of providing any helpful information about the actual problem 😞 .

Now , since bitcoin-core is installed via stable snap channel one could assume a problem with some sort of security confinement. Thus I double checked in snap and bitcoin-core is allowed to modify the user directory (whatever this means in detail). Since I am no expert in snap nor bitcoin I am not really sure how to get to any further.

To reproduce

• Install Ubuntu 20.04
• Install bitcoin-core via snap stable
• Try to start bitcoin-core.daemon /home/user/.bitcoin is preexisting

System information

$ bitcoin-core.daemon -version
Bitcoin Core version v0.20.1

<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->

<!-- What type of machine are you observing the error on (OS/CPU and disk type)? -->

Running a machine with x86_64

To solve permission errors while using the Bitcoin Core snap, see https://snapcraft.io/docs/removable-media-interface . I.e. snap connect bitcoin-core:removable-media or snap connect bitcoin-core:home.

On some systems, snap may not allow access to paths with . as first character.