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218 lines
8.4 KiB
218 lines
8.4 KiB
// Copyright 2021, The Android Open Source Project
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//! This crate provides the PropertyWatcher type, which watches for changes
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//! in Android system properties.
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use anyhow::{anyhow, Context, Result as AnyhowResult};
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use keystore2_system_property_bindgen::prop_info as PropInfo;
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use std::os::raw::c_char;
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use std::ptr::null;
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use std::{
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ffi::{c_void, CStr, CString},
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str::Utf8Error,
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};
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use thiserror::Error;
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/// Errors this crate can generate
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#[derive(Error, Debug)]
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pub enum PropertyWatcherError {
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/// We can't watch for a property whose name contains a NUL character.
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#[error("Cannot convert name to C string")]
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BadNameError(#[from] std::ffi::NulError),
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/// We can only watch for properties that exist when the watcher is created.
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#[error("System property is absent")]
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SystemPropertyAbsent,
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/// __system_property_wait timed out despite being given no timeout.
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#[error("Wait failed")]
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WaitFailed,
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/// read callback was not called
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#[error("__system_property_read_callback did not call callback")]
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ReadCallbackNotCalled,
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/// read callback gave us a NULL pointer
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#[error("__system_property_read_callback gave us a NULL pointer instead of a string")]
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MissingCString,
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/// read callback gave us a bad C string
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#[error("__system_property_read_callback gave us a non-UTF8 C string")]
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BadCString(#[from] Utf8Error),
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/// read callback returned an error
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#[error("Callback failed")]
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CallbackError(#[from] anyhow::Error),
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/// Failure in setting the system property
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#[error("__system_property_set failed.")]
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SetPropertyFailed,
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}
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/// Result type specific for this crate.
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pub type Result<T> = std::result::Result<T, PropertyWatcherError>;
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/// PropertyWatcher takes the name of an Android system property such
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/// as `keystore.boot_level`; it can report the current value of this
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/// property, or wait for it to change.
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pub struct PropertyWatcher {
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prop_name: CString,
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prop_info: *const PropInfo,
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serial: keystore2_system_property_bindgen::__uint32_t,
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}
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impl PropertyWatcher {
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/// Create a PropertyWatcher for the named system property.
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pub fn new(name: &str) -> Result<Self> {
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Ok(Self { prop_name: CString::new(name)?, prop_info: null(), serial: 0 })
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}
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// Lazy-initializing accessor for self.prop_info.
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fn get_prop_info(&mut self) -> Option<*const PropInfo> {
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if self.prop_info.is_null() {
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// Unsafe required for FFI call. Input and output are both const.
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// The returned pointer is valid for the lifetime of the program.
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self.prop_info = unsafe {
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keystore2_system_property_bindgen::__system_property_find(self.prop_name.as_ptr())
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};
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}
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if self.prop_info.is_null() {
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None
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} else {
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Some(self.prop_info)
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}
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}
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fn read_raw(prop_info: *const PropInfo, mut f: impl FnOnce(Option<&CStr>, Option<&CStr>)) {
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// Unsafe function converts values passed to us by
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// __system_property_read_callback to Rust form
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// and pass them to inner callback.
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unsafe extern "C" fn callback(
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res_p: *mut c_void,
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name: *const c_char,
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value: *const c_char,
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_: keystore2_system_property_bindgen::__uint32_t,
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) {
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let name = if name.is_null() { None } else { Some(CStr::from_ptr(name)) };
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let value = if value.is_null() { None } else { Some(CStr::from_ptr(value)) };
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let f = &mut *res_p.cast::<&mut dyn FnMut(Option<&CStr>, Option<&CStr>)>();
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f(name, value);
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}
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let mut f: &mut dyn FnOnce(Option<&CStr>, Option<&CStr>) = &mut f;
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// Unsafe block for FFI call. We convert the FnOnce
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// to a void pointer, and unwrap it in our callback.
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unsafe {
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keystore2_system_property_bindgen::__system_property_read_callback(
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prop_info,
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Some(callback),
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&mut f as *mut _ as *mut c_void,
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)
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}
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}
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/// Call the passed function, passing it the name and current value
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/// of this system property. See documentation for
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/// `__system_property_read_callback` for details.
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/// Returns an error if the property is empty or doesn't exist.
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pub fn read<T, F>(&mut self, f: F) -> Result<T>
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where
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F: FnOnce(&str, &str) -> anyhow::Result<T>,
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{
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let prop_info = self.get_prop_info().ok_or(PropertyWatcherError::SystemPropertyAbsent)?;
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let mut result = Err(PropertyWatcherError::ReadCallbackNotCalled);
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Self::read_raw(prop_info, |name, value| {
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// use a wrapping closure as an erzatz try block.
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result = (|| {
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let name = name.ok_or(PropertyWatcherError::MissingCString)?.to_str()?;
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let value = value.ok_or(PropertyWatcherError::MissingCString)?.to_str()?;
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f(name, value).map_err(PropertyWatcherError::CallbackError)
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})()
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});
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result
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}
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// Waits for the property that self is watching to be created. Returns immediately if the
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// property already exists.
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fn wait_for_property_creation(&mut self) -> Result<()> {
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let mut global_serial = 0;
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loop {
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match self.get_prop_info() {
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Some(_) => return Ok(()),
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None => {
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// Unsafe call for FFI. The function modifies only global_serial, and has
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// no side-effects.
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if !unsafe {
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// Wait for a global serial number change, then try again. On success,
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// the function will update global_serial with the last version seen.
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keystore2_system_property_bindgen::__system_property_wait(
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null(),
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global_serial,
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&mut global_serial,
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null(),
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)
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} {
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return Err(PropertyWatcherError::WaitFailed);
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}
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}
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}
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}
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}
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/// Wait for the system property to change. This
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/// records the serial number of the last change, so
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/// race conditions are avoided.
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pub fn wait(&mut self) -> Result<()> {
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// If the property is null, then wait for it to be created. Subsequent waits will
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// skip this step and wait for our specific property to change.
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if self.prop_info.is_null() {
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return self.wait_for_property_creation();
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}
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let mut new_serial = self.serial;
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// Unsafe block to call __system_property_wait.
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// All arguments are private to PropertyWatcher so we
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// can be confident they are valid.
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if !unsafe {
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keystore2_system_property_bindgen::__system_property_wait(
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self.prop_info,
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self.serial,
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&mut new_serial,
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null(),
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)
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} {
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return Err(PropertyWatcherError::WaitFailed);
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}
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self.serial = new_serial;
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Ok(())
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}
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}
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/// Writes a system property.
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pub fn write(name: &str, value: &str) -> AnyhowResult<()> {
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if
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// Unsafe required for FFI call. Input and output are both const and valid strings.
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unsafe {
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// If successful, __system_property_set returns 0, otherwise, returns -1.
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keystore2_system_property_bindgen::__system_property_set(
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CString::new(name)
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.context("In keystore2::system_property::write: Construction CString from name.")?
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.as_ptr(),
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CString::new(value)
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.context("In keystore2::system_property::write: Constructing CString from value.")?
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.as_ptr(),
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)
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} == 0
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{
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Ok(())
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} else {
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Err(anyhow!(PropertyWatcherError::SetPropertyFailed))
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}
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}
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