feat: add reading and writing PSBT files

The Rust API makes it trivially simple to read and write PSBT to and
from files. Bindings users may make use of this in air-gapped signing
flows. The `PsbtError` already includes an `Io` variant, so we may make
use of that error.

Author: rustaceanrob

bdk-ffi/src/bitcoin.rs

@@ -32,6 +32,8 @@ use bdk_wallet::miniscript::psbt::PsbtExt;
 use bdk_wallet::serde_json;
 
 use std::fmt::Display;
+use std::fs::File;
+use std::io::{BufReader, BufWriter};
 use std::ops::Deref;
 use std::str::FromStr;
 use std::sync::{Arc, Mutex};
@@ -477,6 +479,15 @@ impl Psbt {
         Ok(Psbt(Mutex::new(psbt)))
     }
 
+    /// Create a new `Psbt` from a `.psbt` file.
+    #[uniffi::constructor]
+    pub(crate) fn from_file(path: String) -> Result<Self, PsbtError> {
+        let file = File::open(path)?;
+        let mut buf_read = BufReader::new(file);
+        let psbt: BdkPsbt = BdkPsbt::deserialize_from_reader(&mut buf_read)?;
+        Ok(Psbt(Mutex::new(psbt)))
+    }
+
     /// Serialize the PSBT into a base64-encoded string.
     pub(crate) fn serialize(&self) -> String {
         let psbt = self.0.lock().unwrap().clone();
@@ -548,6 +559,15 @@ impl Psbt {
         }
     }
 
+    /// Write the `Psbt` to a file. Note that the file must not yet exist.
+    pub(crate) fn write_to_file(&self, path: String) -> Result<(), PsbtError> {
+        let file = File::create(path)?;
+        let mut writer = BufWriter::new(file);
+        let psbt = self.0.lock().unwrap();
+        psbt.serialize_to_writer(&mut writer)?;
+        Ok(())
+    }
+
     /// Serializes the PSBT into a JSON string representation.
     pub(crate) fn json_serialize(&self) -> String {
         let psbt = self.0.lock().unwrap();

bdk-ffi/src/error.rs

@@ -1438,6 +1438,22 @@ impl From<BdkPsbtParseError> for PsbtParseError {
     }
 }
 
+impl From<std::io::Error> for PsbtError {
+    fn from(error: std::io::Error) -> Self {
+        PsbtError::Io {
+            error_message: error.to_string(),
+        }
+    }
+}
+
+impl From<bdk_wallet::bitcoin::io::Error> for PsbtError {
+    fn from(error: bdk_wallet::bitcoin::io::Error) -> Self {
+        PsbtError::Io {
+            error_message: error.to_string(),
+        }
+    }
+}
+
 impl From<BdkPsbtFinalizeError> for PsbtFinalizeError {
     fn from(value: BdkPsbtFinalizeError) -> Self {
         match value {