Implement configuration handling

This patch implements basic configuration handling that reads a
configuration file and stores the parsed data in the ExecCtx and RunCtx
structs.  It supports three configuration items:
- model (previously only --model)
- no_cache (previously only NITROCLI_NO_CACHE)
- verbosity (previously only --verbose)

9 files changed, 167 insertions, 28 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bdca896..21b5f09 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,8 @@
 Unreleased
 ----------
+- Added support for configuration files
+  - Added `toml` dependency in version `0.5.6`
+  - Added `serde` dependency in version `1.0.114`
 - Changed default OTP format from `hex` to `base32`
 - Improved error reporting format and fidelity
   - Added `anyhow` dependency in version `1.0.32`
diff --git a/Cargo.lock b/Cargo.lock
index 810b868..22c764b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -99,7 +99,9 @@ dependencies = [
  "nitrokey-test",
  "nitrokey-test-state",
  "regex",
+ "serde",
  "structopt",
+ "toml",
 ]
 
 [[package]]
@@ -212,6 +214,26 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7fe5bd57d1d7414c6b5ed48563a2c855d995ff777729dcd91c369ec7fea395ae"
 
 [[package]]
+name = "serde"
+version = "1.0.114"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5317f7588f0a5078ee60ef675ef96735a1442132dc645eb1d12c018620ed8cd3"
+dependencies = [
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_derive"
+version = "1.0.114"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2a0be94b04690fbaed37cddffc5c134bf537c8e3329d53e982fe04c374978f8e"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
 name = "structopt"
 version = "0.3.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -276,6 +298,15 @@ dependencies = [
 ]
 
 [[package]]
+name = "toml"
+version = "0.5.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ffc92d160b1eef40665be3a05630d003936a3bc7da7421277846c2613e92c71a"
+dependencies = [
+ "serde",
+]
+
+[[package]]
 name = "unicode-segmentation"
 version = "1.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
diff --git a/Cargo.toml b/Cargo.toml
index 961e82f..d652cba 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -58,10 +58,17 @@ version = "0.2"
 [dependencies.nitrokey]
 version = "0.7.1"
 
+[dependencies.serde]
+version = "1.0"
+features = ["derive"]
+
 [dependencies.structopt]
 version = "0.3.7"
 default-features = false
 
+[dependencies.toml]
+version = "0.5.6"
+
 [dev-dependencies.nitrokey-test]
 version = "0.4"
 
diff --git a/src/args.rs b/src/args.rs
index 0f548e4..7f2dc31 100644
--- a/src/args.rs
+++ b/src/args.rs
@@ -18,7 +18,7 @@
 // *************************************************************************
 
 /// Provides access to a Nitrokey device
-#[derive(structopt::StructOpt)]
+#[derive(Debug, structopt::StructOpt)]
 #[structopt(name = "nitrocli")]
 pub struct Args {
   /// Increases the log level (can be supplied multiple times)
@@ -57,6 +57,19 @@ impl From<DeviceModel> for nitrokey::Model {
   }
 }
 
+impl<'de> serde::Deserialize<'de> for DeviceModel {
+  fn deserialize<D>(deserializer: D) -> Result<DeviceModel, D::Error>
+  where
+    D: serde::Deserializer<'de>,
+  {
+    use serde::de::Error as _;
+    use std::str::FromStr as _;
+
+    let s = String::deserialize(deserializer)?;
+    DeviceModel::from_str(&s).map_err(D::Error::custom)
+  }
+}
+
 Command! {
   /// A top-level command for nitrocli.
   Command, [
diff --git a/src/commands.rs b/src/commands.rs
index 1d59af5..090d532 100644
--- a/src/commands.rs
+++ b/src/commands.rs
@@ -39,7 +39,7 @@ use crate::ExecCtx;
 
 /// Set `libnitrokey`'s log level based on the execution context's verbosity.
 fn set_log_level(ctx: &mut ExecCtx<'_>) {
-  let log_lvl = match ctx.verbosity {
+  let log_lvl = match ctx.config.verbosity {
     // The error log level is what libnitrokey uses by default. As such,
     // there is no harm in us setting that as well when the user did not
     // ask for higher verbosity.
@@ -63,7 +63,7 @@ where
 
   set_log_level(ctx);
 
-  let device = match ctx.model {
+  let device = match ctx.config.model {
     Some(model) => manager.connect_model(model.into()).with_context(|| {
       anyhow::anyhow!("Nitrokey {} device not found", model.as_user_facing_str())
     })?,
@@ -83,7 +83,7 @@ where
 
   set_log_level(ctx);
 
-  if let Some(model) = ctx.model {
+  if let Some(model) = ctx.config.model {
     if model != args::DeviceModel::Storage {
       anyhow::bail!("This command is only available on the Nitrokey Storage");
     }
diff --git a/src/config.rs b/src/config.rs
new file mode 100644
index 0000000..850d217
--- /dev/null
+++ b/src/config.rs
@@ -0,0 +1,70 @@
+// config.rs
+
+// *************************************************************************
+// * Copyright (C) 2020 Daniel Mueller (deso@posteo.net)                   *
+// *                                                                       *
+// * This program is free software: you can redistribute it and/or modify  *
+// * it under the terms of the GNU General Public License as published by  *
+// * the Free Software Foundation, either version 3 of the License, or     *
+// * (at your option) any later version.                                   *
+// *                                                                       *
+// * This program is distributed in the hope that it will be useful,       *
+// * but WITHOUT ANY WARRANTY; without even the implied warranty of        *
+// * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
+// * GNU General Public License for more details.                          *
+// *                                                                       *
+// * You should have received a copy of the GNU General Public License     *
+// * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+// *************************************************************************
+
+use std::fs;
+use std::path;
+
+use crate::args;
+
+use anyhow::Context as _;
+
+/// The configuration for nitrocli, usually read from configuration
+/// files and environment variables.
+#[derive(Clone, Copy, Debug, Default, PartialEq, serde::Deserialize)]
+pub struct Config {
+  /// The model to connect to.
+  pub model: Option<args::DeviceModel>,
+  /// Whether to bypass the cache for all secrets or not.
+  #[serde(default)]
+  pub no_cache: bool,
+  /// The log level.
+  #[serde(default)]
+  pub verbosity: u8,
+}
+
+impl Config {
+  pub fn load() -> anyhow::Result<Self> {
+    load_user_config().map(|o| o.unwrap_or_default())
+  }
+
+  pub fn update(&mut self, args: &args::Args) {
+    if args.model.is_some() {
+      self.model = args.model;
+    }
+    if args.verbose > 0 {
+      self.verbosity = args.verbose;
+    }
+  }
+}
+
+fn load_user_config() -> anyhow::Result<Option<Config>> {
+  let path = path::Path::new("config.toml");
+  if path.is_file() {
+    read_config_file(&path).map(Some)
+  } else {
+    Ok(None)
+  }
+}
+
+pub fn read_config_file(path: &path::Path) -> anyhow::Result<Config> {
+  let s = fs::read_to_string(path)
+    .with_context(|| format!("Failed to read configuration file '{}'", path.display()))?;
+  toml::from_str(&s)
+    .with_context(|| format!("Failed to parse configuration file '{}'", path.display()))
+}
diff --git a/src/main.rs b/src/main.rs
index a8c0a46..3535a91 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -69,6 +69,7 @@ mod arg_util;
 
 mod args;
 mod commands;
+mod config;
 mod pinentry;
 #[cfg(test)]
 mod tests;
@@ -108,8 +109,6 @@ where
 /// the command execution.
 #[allow(missing_debug_implementations)]
 pub struct ExecCtx<'io> {
-  /// The Nitrokey model to use.
-  pub model: Option<args::DeviceModel>,
   /// See `RunCtx::stdout`.
   pub stdout: &'io mut dyn io::Write,
   /// See `RunCtx::stderr`.
@@ -124,10 +123,8 @@ pub struct ExecCtx<'io> {
   pub new_user_pin: Option<ffi::OsString>,
   /// See `RunCtx::password`.
   pub password: Option<ffi::OsString>,
-  /// See `RunCtx::no_cache`.
-  pub no_cache: bool,
-  /// The verbosity level to use for logging.
-  pub verbosity: u64,
+  /// See `RunCtx::config`.
+  pub config: config::Config,
 }
 
 impl<'io> Stdio for ExecCtx<'io> {
@@ -142,8 +139,9 @@ fn handle_arguments(ctx: &mut RunCtx<'_>, args: Vec<String>) -> anyhow::Result<(
 
   match args::Args::from_iter_safe(args.iter()) {
     Ok(args) => {
+      let mut config = ctx.config;
+      config.update(&args);
       let mut ctx = ExecCtx {
-        model: args.model,
         stdout: ctx.stdout,
         stderr: ctx.stderr,
         admin_pin: ctx.admin_pin.take(),
@@ -151,8 +149,7 @@ fn handle_arguments(ctx: &mut RunCtx<'_>, args: Vec<String>) -> anyhow::Result<(
         new_admin_pin: ctx.new_admin_pin.take(),
         new_user_pin: ctx.new_user_pin.take(),
         password: ctx.password.take(),
-        no_cache: ctx.no_cache,
-        verbosity: args.verbose.into(),
+        config,
       };
       args.cmd.execute(&mut ctx)
     }
@@ -187,8 +184,9 @@ pub(crate) struct RunCtx<'io> {
   pub new_user_pin: Option<ffi::OsString>,
   /// A password used by some commands, if provided through an environment variable.
   pub password: Option<ffi::OsString>,
-  /// Whether to bypass the cache for all secrets or not.
-  pub no_cache: bool,
+  /// The configuration, usually read from configuration files and environment
+  /// variables.
+  pub config: config::Config,
 }
 
 fn run<'ctx, 'io: 'ctx>(ctx: &'ctx mut RunCtx<'io>, args: Vec<String>) -> i32 {
@@ -206,19 +204,33 @@ fn main() {
 
   let mut stdout = io::stdout();
   let mut stderr = io::stderr();
-  let args = env::args().collect::<Vec<_>>();
-  let ctx = &mut RunCtx {
-    stdout: &mut stdout,
-    stderr: &mut stderr,
-    admin_pin: env::var_os(NITROCLI_ADMIN_PIN),
-    user_pin: env::var_os(NITROCLI_USER_PIN),
-    new_admin_pin: env::var_os(NITROCLI_NEW_ADMIN_PIN),
-    new_user_pin: env::var_os(NITROCLI_NEW_USER_PIN),
-    password: env::var_os(NITROCLI_PASSWORD),
-    no_cache: env::var_os(NITROCLI_NO_CACHE).is_some(),
+
+  let rc = match config::Config::load() {
+    Ok(mut config) => {
+      if env::var_os(NITROCLI_NO_CACHE).is_some() {
+        config.no_cache = true;
+      }
+
+      let args = env::args().collect::<Vec<_>>();
+      let ctx = &mut RunCtx {
+        stdout: &mut stdout,
+        stderr: &mut stderr,
+        admin_pin: env::var_os(NITROCLI_ADMIN_PIN),
+        user_pin: env::var_os(NITROCLI_USER_PIN),
+        new_admin_pin: env::var_os(NITROCLI_NEW_ADMIN_PIN),
+        new_user_pin: env::var_os(NITROCLI_NEW_USER_PIN),
+        password: env::var_os(NITROCLI_PASSWORD),
+        config,
+      };
+
+      run(ctx, args)
+    }
+    Err(err) => {
+      let _ = writeln!(stderr, "{:?}", err);
+      1
+    }
   };
 
-  let rc = run(ctx, args);
   // We exit the process the hard way below. The problem is that because
   // of this, buffered IO may not be flushed. So make sure to explicitly
   // flush before exiting. Note that stderr is unbuffered, alleviating
diff --git a/src/pinentry.rs b/src/pinentry.rs
index 510d7b0..f538a47 100644
--- a/src/pinentry.rs
+++ b/src/pinentry.rs
@@ -238,7 +238,7 @@ where
 {
   let cache_id = entry
     .cache_id()
-    .and_then(|id| if ctx.no_cache { None } else { Some(id) })
+    .and_then(|id| if ctx.config.no_cache { None } else { Some(id) })
     // "X" is a sentinel value indicating that no caching is desired.
     .unwrap_or_else(|| "X".into())
     .into();
diff --git a/src/tests/mod.rs b/src/tests/mod.rs
index f26ab80..9477964 100644
--- a/src/tests/mod.rs
+++ b/src/tests/mod.rs
@@ -115,7 +115,10 @@ impl Nitrocli {
       new_admin_pin: self.new_admin_pin.clone(),
       new_user_pin: self.new_user_pin.clone(),
       password: self.password.clone(),
-      no_cache: true,
+      config: crate::config::Config {
+        no_cache: true,
+        ..Default::default()
+      },
     };
 
     (f(ctx, args), stdout, stderr)