script: Add fish completions

Completions are dynamically generated from the respective binary help
pages.

Completions should be sourced into the shell or added to
$XDG_CONFIG/fish/completions.

contrib/completions/fish/bitcoin-cli.fish

@@ -0,0 +1,99 @@
+# Disable files from being included in completions by default
+complete --command bitcoin-cli --no-files
+
+function __fish_bitcoin_cli_get_commands_helper
+    set --local cmd (commandline -oc)
+
+    # Don't return commands if '-help or -?' in commandline
+    if string match --quiet --regex -- '^-help$|^-\?$' $cmd
+        return
+    end
+
+    # Strip help cmd from token to avoid duplication errors
+    set --local cmd (string match --invert --regex -- '^help$' $cmd)
+    # Strip -stdin* options to avoid waiting for input while we fetch completions
+    # TODO: this appears to be broken when run as tab completion (requires ctrl+c to exit)
+    set --local cmd (string match --invert --regex -- '^-stdin.*$' $cmd)
+
+    # Match, format and return commands
+    for command in ($cmd help 2>&1 | string match --invert -r '^\=\=.*' | string match --invert -r '^\\s*$')
+        echo $command
+    end
+end
+
+function __fish_bitcoin_cli_get_commands
+    argparse 'nohelp' 'commandsonly' -- $argv
+    set --local commands
+
+    # Exclude description, exclude help
+    if set -q _flag_nohelp; and set -q _flag_commandsonly
+        set --append commands (__fish_bitcoin_cli_get_commands_helper | string replace -r ' .*$' '' | string match --invert -r 'help')
+    # Include description, exclude help
+    else if set -q _flag_nohelp
+        set --append commands (__fish_bitcoin_cli_get_commands_helper | string replace ' ' \t | string match --invert -r 'help')
+    # Exclude description, include help
+    else if set -q _flag_commandsonly
+        set --append commands (__fish_bitcoin_cli_get_commands_helper | string replace -r ' .*$' '')
+    # Include description, include help
+    else
+        set --append commands (__fish_bitcoin_cli_get_commands_helper | string replace ' ' \t)
+    end
+
+    if string match -q -r '^.*error.*$' $commands[1]
+        # RPC offline or RPC wallet not loaded
+        return
+    else
+        for command in $commands
+            echo $command
+        end
+    end
+end
+
+
+function __fish_bitcoin_cli_get_options
+    argparse 'nofiles' -- $argv
+    set --local cmd (commandline -oc)
+    # Don't return options if '-help or -?' in commandline
+    if string match --quiet --regex -- '^-help$|-\?$' $cmd
+        return
+    end
+    set --local options
+
+    if set -q _flag_nofiles
+        set --append options ($cmd -help 2>&1 | string match -r '^  -.*' | string replace -r '  -' '-' | string replace -r '=.*' '=' | string match --invert -r '^.*=$')
+    else
+        set --append options ($cmd -help 2>&1 | string match -r '^  -.*' | string replace -r '  -' '-' | string replace -r '=.*' '=' | string match -r '^.*=$')
+    end
+
+    for option in $options
+        echo $option
+    end
+end
+
+# Add options with file completion
+# Don't offer after a command is given
+complete \
+    --command bitcoin-cli \
+    --no-files \
+    --condition "not __fish_seen_subcommand_from (__fish_bitcoin_cli_get_commands --commandsonly)" \
+    --arguments "(__fish_bitcoin_cli_get_options)"
+# Enable file completions only if the commandline now contains a `*.=` style option
+complete --command bitcoin-cli \
+    --condition 'string match --regex -- ".*=" (commandline -pt)' \
+    --force-files
+
+# Add options without file completion
+# Don't offer after a command is given
+complete \
+    --command bitcoin-cli \
+    --no-files \
+    --condition "not __fish_seen_subcommand_from (__fish_bitcoin_cli_get_commands --commandsonly)" \
+    --arguments "(__fish_bitcoin_cli_get_options --nofiles)"
+
+# Add commands
+# Permit command completions after `bitcoin-cli help` but not after other commands
+complete \
+    --command bitcoin-cli \
+    --no-files \
+    --condition "not __fish_seen_subcommand_from (__fish_bitcoin_cli_get_commands --commandsonly --nohelp)" \
+    --arguments "(__fish_bitcoin_cli_get_commands)"