[ Setup ]
Assuming a working bitcoin core setup is present and all paths (data, conf, run) are properly setup as required by the service unit provided here (https://github.com/bitcoin/bitcoin/blob/master/contrib/init/bitcoind.service).

[ Issue ]
If the following statements are true:

- the folder `/etc/bitcoin` has the permissions `0710` and has the owner/group `root:bitcoin`
- the config file has permissions or ownership that make it not accessible to the user running the daemon, for example: `/etc/bitcoin/bitcoin.conf` has the permissions `0640` and has the owner/group `root:root`.

Starting the service will result in an instance using plain defaults without a single warning about the specified config file being inaccessible to it.

[ Expected behavior ]
Startup failure with a clear error message that the provided config file is not readable. I can't think of a situation in which a config file is specified explicitly (in the startup options, as per service unit linked above), but inaccessible, where the fail condition should be to keep booting using defaults instead.

[ Actual behavior ]
The service starts with no warning whatsoever, nor a message logged about it (see debug.log below)

[ Reliability ]
100% reproducible

[ Core version ]
0.21.1

[ OS ]
Ubuntu server 20.04.1

[ Bootstrap Log ]
```2021-08-02T20:54:00Z Bitcoin Core version v0.21.1 (release build)
2021-08-02T20:54:00Z Assuming ancestors of block 0000000000000000000b9d2ec5a352ecba0592946514a92f14319dc2b367fc72 have valid signatures.
2021-08-02T20:54:00Z Setting nMinimumChainWork=00000000000000000000000000000000000000001533efd8d716a517fe2c5008
2021-08-02T20:54:00Z Using the 'sse4(1way),sse41(4way),avx2(8way)' SHA256 implementation
2021-08-02T20:54:00Z Using RdSeed as additional entropy source
2021-08-02T20:54:00Z Using RdRand as an additional entropy source
2021-08-02T20:54:00Z Default data directory /var/lib/bitcoind/.bitcoin
2021-08-02T20:54:00Z Using data directory /var/lib/bitcoind
2021-08-02T20:54:00Z Config file: /etc/bitcoin/bitcoind.conf
2021-08-02T20:54:00Z Command-line arg: conf="/etc/bitcoin/bitcoind.conf"
2021-08-02T20:54:00Z Command-line arg: daemon=""
2021-08-02T20:54:00Z Command-line arg: datadir="/var/lib/bitcoind"
2021-08-02T20:54:00Z Command-line arg: pid="/run/bitcoind/bitcoind.pid"
2021-08-02T20:54:00Z Using at most 125 automatic connections (1024 file descriptors available)
2021-08-02T20:54:00Z Using 16 MiB out of 32/2 requested for signature cache, able to store 524288 elements
2021-08-02T20:54:01Z Using 16 MiB out of 32/2 requested for script execution cache, able to store 524288 elements
2021-08-02T20:54:01Z Script verification uses 7 additional threads
2021-08-02T20:54:01Z scheduler thread start
2021-08-02T20:54:01Z HTTP: creating work queue of depth 16
2021-08-02T20:54:01Z Using random cookie authentication.
2021-08-02T20:54:01Z Generated RPC authentication cookie /var/lib/bitcoind/.cookie
2021-08-02T20:54:01Z HTTP: starting 4 worker threads
2021-08-02T20:54:01Z Using wallet directory /var/lib/bitcoind
2021-08-02T20:54:01Z init message: Verifying wallet(s)...
2021-08-02T20:54:01Z init message: Loading banlist...
2021-08-02T20:54:01Z SetNetworkActive: true
2021-08-02T20:54:01Z Using /16 prefix for IP bucketing
2021-08-02T20:54:01Z Cache configuration:
2021-08-02T20:54:01Z * Using 2.0 MiB for block index database
2021-08-02T20:54:01Z * Using 8.0 MiB for chain state database
2021-08-02T20:54:01Z * Using 440.0 MiB for in-memory UTXO set (plus up to 286.1 MiB of unused mempool space)
2021-08-02T20:54:01Z init message: Loading block index...
2021-08-02T20:54:01Z Switching active chainstate to Chainstate [ibd] @ height -1 (null)
2021-08-02T20:54:01Z Opening LevelDB in /var/lib/bitcoind/blocks/index
2021-08-02T20:54:01Z Opened LevelDB successfully
2021-08-02T20:54:01Z Using obfuscation key for /var/lib/bitcoind/blocks/index: 0000000000000000
2021-08-02T20:54:04Z LoadBlockIndexDB: last block file = 2673
2021-08-02T20:54:04Z LoadBlockIndexDB: last block file info: CBlockFileInfo(blocks=95, size=126973024, heights=693816...693910, time=2021-08-02...2021-08-02)
2021-08-02T20:54:04Z Checking all blk files are present...
2021-08-02T20:54:04Z Opening LevelDB in /var/lib/bitcoind/chainstate
2021-08-02T20:54:04Z Opened LevelDB successfully
2021-08-02T20:54:04Z Using obfuscation key for /var/lib/bitcoind/chainstate: 552509bc3d6a34b5
2021-08-02T20:54:04Z Loaded best chain: hashBestChain=0000000000000000000275f0f28b4c0cc7a7552f78669b2f6ba2b3d9809d065b height=693910 date=2021-08-02T20:24:59Z progress=0.999991
2021-08-02T20:54:04Z init message: Rewinding blocks...
2021-08-02T20:54:04Z FlushStateToDisk: write coins cache to disk (0 coins, 0kB) started
2021-08-02T20:54:04Z FlushStateToDisk: write coins cache to disk (0 coins, 0kB) completed (0.00s)
2021-08-02T20:54:04Z init message: Verifying blocks...
2021-08-02T20:54:04Z Verifying last 6 blocks at level 3
2021-08-02T20:54:04Z [0%]...[16%]...[33%]...[50%]...[66%]...[83%]...[99%]...[DONE].
2021-08-02T20:54:05Z No coin database inconsistencies in last 6 blocks (9363 transactions)
2021-08-02T20:54:05Z  block index            4042ms
2021-08-02T20:54:05Z block tree size = 693913
2021-08-02T20:54:05Z nBestHeight = 693910
2021-08-02T20:54:05Z loadblk thread start
2021-08-02T20:54:05Z Bound to [::]:8333
2021-08-02T20:54:05Z torcontrol thread start
2021-08-02T20:54:05Z Bound to 0.0.0.0:8333
2021-08-02T20:54:05Z Bound to 127.0.0.1:8334
2021-08-02T20:54:05Z init message: Loading P2P addresses...
2021-08-02T20:54:05Z Leaving InitialBlockDownload (latching to false)
2021-08-02T20:54:05Z Loaded 65864 addresses from peers.dat  174ms
2021-08-02T20:54:05Z Loaded 2 addresses from "anchors.dat"
2021-08-02T20:54:05Z 2 block-relay-only anchors will be tried for connections.
2021-08-02T20:54:05Z init message: Starting network threads...
2021-08-02T20:54:05Z net thread start
2021-08-02T20:54:05Z addcon thread start
2021-08-02T20:54:05Z dnsseed thread start
2021-08-02T20:54:05Z Waiting 300 seconds before querying DNS seeds.
2021-08-02T20:54:05Z init message: Done loading
2021-08-02T20:54:05Z opencon thread start
2021-08-02T20:54:05Z msghand thread start
2021-08-02T20:54:05Z New outbound peer connected: version: 70016, blocks=693910, peer=0 (block-relay)
2021-08-02T20:54:07Z New outbound peer connected: version: 70015, blocks=693910, peer=1 (block-relay)
2021-08-02T20:54:07Z Imported mempool transactions from disk: 4395 succeeded, 0 failed, 0 expired, 0 already there, 0 waiting for initial broadcast
2021-08-02T20:54:07Z loadblk thread exit
2021-08-02T20:54:07Z New outbound peer connected: version: 70015, blocks=693910, peer=2 (full-relay)
2021-08-02T20:54:13Z New outbound peer connected: version: 70016, blocks=693910, peer=3 (full-relay)
2021-08-02T20:54:13Z New outbound peer connected: version: 70016, blocks=693910, peer=4 (full-relay)
2021-08-02T20:54:16Z P2P peers available. Skipped DNS seeding.
2021-08-02T20:54:16Z dnsseed thread exit
2021-08-02T20:54:20Z New outbound peer connected: version: 70015, blocks=693910, peer=5 (full-relay)```

{
    return m_network == CBaseChainParams::MAIN || m_network_only_args.count(arg) == 0;
}
util::SettingsValue ArgsManager::GetSetting(const std::string& arg) const
{
    LOCK(cs_args);
    return util::GetSetting(
        m_settings, m_network, SettingName(arg), !UseDefaultSection(arg),  false);
}
std::vector<util::SettingsValue> ArgsManager::GetSettingsList(const std::string& arg) const
{
    LOCK(cs_args);
    return util::GetSettingsList(m_settings, m_network, SettingName(arg), !UseDefaultSection(arg));
}
void ArgsManager::logArgsPrefix(