Unclear error message in fdbbackup if the backup url uses a symlink

[oleg68]

Unclear error message in fdbbackup if the backup url uses a symlink

Problem statement

If a file backup url usesng a symlink then fdbbackup fails with an unclear error message 'Disk i/o operation failed'

Steps to reproduce

1. Create a FDB cluster
2. Create a symlink file to a backup location
3. Start fdbbackup using the symlink file as the backup destination

fdbbackup start -C fdb-primary.cluster -d file:///mnt/backup/fdb/hot/9

Expected result

Starting backup progress or a clear error message why it can not start

Actual result

ERROR: 'Disk i/o operation failed' on URL 'file:///mnt/backup/fdb/hot/9'

Findings

The BackupContainerLocalDirectory constructor compares the url file path with the it's absolute path and throws io_error if these paths are different. I don't know the reason of this checking, but the exception thrown does not contain any useful information about the failed check

Proposal

1. Put a clear informational message to IBackupContainer::lastOpenError
2. Throw backup_invalid_url instead of io_error for forcing fdbbackup exception handler to use IBackupContainer::lastOpenError

This pull request implements this proposal. The new error message is

ERROR: 'Backup Container URL invalid' on URL 'file:///mnt/backup/fdb/hot/9': Backup path '/mnt/backup/fdb/hot/9' must be the absolute path '/mnt/synology/nfs/backup/fdb/hot/9'

[jzhou77]

@fdb-build ok to test

[oleg68]

Thanks